Add support for aarch64 builds

[thp]

This doesn't work yet and has not been tested, but GCC already supports aarch64 (obviously), and there's an aarch64 build of e.g. Windows 11 that runs on Parallels for Mac. I guess don't merge yet until this has been verified working, but since I wrote it for testing (didn't end up working...) I thought I might throw this in here before just removing the test code :)

[thp]

For the record, the error I'm getting when building binutils:

$HOME/.zeranoe/mingw-w64/src/binutils/isl/include/isl/cpp.h:4237:68: error: no member named 'on_error' in 'isl::exception'
  options_scoped_set_on_error saved_on_error(saved_ctx, exception::on_error);
                                                        ~~~~~~~~~~~^

[thp]

Ok, the particular error above is fixed by adding -std=c++14 to the CXXFLAGS when building with clang.

But now binutils fails with this:

checking langinfo.h presence... configure: error: GAS does not know what format to use for target aarch64-w64-mingw32

That points to binutils probably haven't been "ported" to 64-bit ARM Windows.

[Zeranoe]

Closing for now until this is ready.