Add RISC-V RVV implementation

[WoWaster]

Add RVV support. Current implementation targets both RVV 0.7.1 and RVV 1.0, this is bad for two reason:

• we need some workarounds with macros' because of __riscv prefix for intrinsics in newer compilers
• we need more instructions because some useful ones were unavailable in RVV 0.7.1

RVV 1.0 can only be tested with QEMU because there is no boards with up-to-date vectors. QEMU also cannot be used for performance testing because it executes all vector ops on scalar registers.

I've tested RVV 0.7.1 on Lichee Pi4A using cross-compilation with Xuantie GCC toolchain. Default xxhsum -b results were uninformative for me, so here's full benchmarks.
With -DXXH_VECTOR=XXH_SCALAR:

➜ ./xxhsum --benchmark-all
xxhsum 0.8.2 by Yann Collet
compiled as 64-bit riscv little endian with GCC 10.2.0
Sample of 100 KB...
 1#XXH32                         :     102400 ->    17771 it/s ( 1735.5 MB/s)
 2#XXH32 unaligned               :     102400 ->     9218 it/s (  900.2 MB/s)
 3#XXH64                         :     102400 ->    29864 it/s ( 2916.4 MB/s)
 4#XXH64 unaligned               :     102400 ->    11421 it/s ( 1115.3 MB/s)
 5#XXH3_64b                      :     102400 ->     6608 it/s (  645.3 MB/s)
 6#XXH3_64b unaligned            :     102400 ->     6613 it/s (  645.8 MB/s)
 7#XXH3_64b w/seed               :     102400 ->     6613 it/s (  645.8 MB/s)
 8#XXH3_64b w/seed unaligned     :     102400 ->     6615 it/s (  646.0 MB/s)
 9#XXH3_64b w/secret             :     102400 ->     2671 it/s (  260.8 MB/s)
10#XXH3_64b w/secret unaligned   :     102400 ->     2669 it/s (  260.7 MB/s)
11#XXH128                        :     102400 ->     6597 it/s (  644.2 MB/s)
12#XXH128 unaligned              :     102400 ->     6607 it/s (  645.2 MB/s)
13#XXH128 w/seed                 :     102400 ->     6605 it/s (  645.0 MB/s)
14#XXH128 w/seed unaligned       :     102400 ->     6611 it/s (  645.6 MB/s)
15#XXH128 w/secret               :     102400 ->     2668 it/s (  260.5 MB/s)
16#XXH128 w/secret unaligned     :     102400 ->     2668 it/s (  260.6 MB/s)
17#XXH32_stream                  :     102400 ->     7556 it/s (  737.8 MB/s)
18#XXH32_stream unaligned        :     102400 ->     7646 it/s (  746.7 MB/s)
19#XXH64_stream                  :     102400 ->    10274 it/s ( 1003.3 MB/s)
20#XXH64_stream unaligned        :     102400 ->    10234 it/s (  999.4 MB/s)
21#XXH3_stream                   :     102400 ->     2732 it/s (  266.8 MB/s)
22#XXH3_stream unaligned         :     102400 ->     2724 it/s (  266.0 MB/s)
23#XXH3_stream w/seed            :     102400 ->     2728 it/s (  266.4 MB/s)
24#XXH3_stream w/seed unaligned  :     102400 ->     2720 it/s (  265.6 MB/s)
25#XXH128_stream                 :     102400 ->     2730 it/s (  266.6 MB/s)
26#XXH128_stream unaligned       :     102400 ->     2724 it/s (  266.0 MB/s)
27#XXH128_stream w/seed          :     102400 ->     2726 it/s (  266.2 MB/s)
28#XXH128_stream w/seed unaligne :     102400 ->     2719 it/s (  265.5 MB/s)

With -DXXH_VECTOR=XXH_RVV:

➜ ./xxhsum --benchmark-all -i10
xxhsum 0.8.2 by Yann Collet
compiled as 64-bit riscv little endian with GCC 10.2.0
Sample of 100 KB...
 1#XXH32                         :     102400 ->    17779 it/s ( 1736.2 MB/s)
 2#XXH32 unaligned               :     102400 ->     8977 it/s (  876.7 MB/s)
 3#XXH64                         :     102400 ->    32211 it/s ( 3145.6 MB/s)
 4#XXH64 unaligned               :     102400 ->     9707 it/s (  947.9 MB/s)
 5#XXH3_64b                      :     102400 ->     4840 it/s (  472.6 MB/s)
 6#XXH3_64b unaligned            :     102400 ->     4676 it/s (  456.6 MB/s)
 7#XXH3_64b w/seed               :     102400 ->     4717 it/s (  460.6 MB/s)
 8#XXH3_64b w/seed unaligned     :     102400 ->     4620 it/s (  451.2 MB/s)
 9#XXH3_64b w/secret             :     102400 ->     4521 it/s (  441.5 MB/s)
10#XXH3_64b w/secret unaligned   :     102400 ->     4420 it/s (  431.7 MB/s)
11#XXH128                        :     102400 ->     4759 it/s (  464.7 MB/s)
12#XXH128 unaligned              :     102400 ->     4651 it/s (  454.2 MB/s)
13#XXH128 w/seed                 :     102400 ->     4671 it/s (  456.2 MB/s)
14#XXH128 w/seed unaligned       :     102400 ->     4564 it/s (  445.7 MB/s)
15#XXH128 w/secret               :     102400 ->     4428 it/s (  432.4 MB/s)
16#XXH128 w/secret unaligned     :     102400 ->     4335 it/s (  423.3 MB/s)
17#XXH32_stream                  :     102400 ->     7542 it/s (  736.5 MB/s)
18#XXH32_stream unaligned        :     102400 ->     7630 it/s (  745.2 MB/s)
19#XXH64_stream                  :     102400 ->    10239 it/s (  999.9 MB/s)
20#XXH64_stream unaligned        :     102400 ->    10217 it/s (  997.7 MB/s)
21#XXH3_stream                   :     102400 ->     4910 it/s (  479.5 MB/s)
22#XXH3_stream unaligned         :     102400 ->     4737 it/s (  462.6 MB/s)
23#XXH3_stream w/seed            :     102400 ->     4908 it/s (  479.3 MB/s)
24#XXH3_stream w/seed unaligned  :     102400 ->     4730 it/s (  461.9 MB/s)
25#XXH128_stream                 :     102400 ->     4906 it/s (  479.1 MB/s)
26#XXH128_stream unaligned       :     102400 ->     4732 it/s (  462.1 MB/s)
27#XXH128_stream w/seed          :     102400 ->     4903 it/s (  478.8 MB/s)
28#XXH128_stream w/seed unaligne :     102400 ->     4726 it/s (  461.5 MB/s)

As soon as I get a board with RVV 1.0, I'll try to adjust code and benchmark it, but for now let it be a draft.