Default exec=target assumptions on toolchains are too general when trying to use custom toolchains

[illicitonion]

The musl_cross_compiling example doesn't work out of the box when compiling on linux-gnu for linux-musl.

In that case, we need two rust toolchains - one for our exec tools which compiles linux-gnu -> linux gnu, and another for our target output which compiles linux-gnu -> linux-musl. When doing a build currently, the correct C++ toolchain is chosen in each case, but the linux-gnu -> linux-gnu Rust toolchain is selected for both target platforms. This is incorrect - the linux-gnu -> linux-gnu toolchain should not be selected for linux-gnu -> linux-musl because it has a constraint it doesn't satisfy.

I believe this is because in a few places we hard-code an assumption basically that "any Linux toolchain is good enough for any Linux target".

One of these places is here:

rules_rust/rust/repositories.bzl

Lines 984 to 988 in 8b977b5

	if toolchain.target_triple == exec_triple:
	# The exec triple implicitly gets a toolchain with itself as a target - use default_target_compatible_with for it
	target_compatible_with = default_target_compatible_with
	elif type(extra_target_triples) == "dict":
	target_compatible_with = extra_target_triples.get(toolchain.target_triple)

where we prevent users from telling us "this toolchain actually has a more specific target platform than you expect" - we should swap the order of these if-branches.

Another is here:

rules_rust/rust/repositories.bzl

Line 870 in 8b977b5

for target_triple in depset([exec_triple] + extra_target_triples).to_list():

where we encode an assumption that every toolchain is appropriate for targeting its own exec triple.

Fixing these two is sufficient for building example that depends on libc.

See https://github.com/illicitonion/rules_rust/commits/musl-from-linux/ for an example of making this work - the top commit does some hacky changes to break these assumptions, but someone who knows more about the toolchain set-up and/or has more time to think about the consequences of these changes should work out what the best real fix for these problems is.

cc @matte1