Export and use CROSS_TOOLCHAIN_PREFIX.

[Alexhuszagh]

No description provided.

[Alexhuszagh]

This will block #1112, #1118, #1123, and #1125 since it touches so many files, but simplifies a lot of the CMake files, as well as the linker files in #1125. This lets us get rid of a few effectively redundant scripts (like linkers for ARMv5TE, MIPS64, and MIPS64EL musl all of which checked for Rust 1.65 for linker flags), since we can now use CROSS_TOOLCHAIN_PREFIX for everything instead. Since the prefix contains the trailing -, so for example, riscv64-linux-gnu-, native toolchains can just use an empty variable so for every GCC target the proper C compiler can be found with ${CROSS_TOOLCHAIN_PREFIX}gcc.

bors try

[bors]

try

Build failed:

• conclusion

[Alexhuszagh]

Windows and executable permissions strikes again :/
bors try --target *freebsd

[bors]

try

Build succeeded:

• conclusion

[Alexhuszagh]

bors try

[bors]

try

Build succeeded:

• conclusion

[Emilgardis]

nice

[Emilgardis]

do we need this to be defined as empty?

[Emilgardis]

ditto, do we need this?

[Alexhuszagh]

It ensures consistency in all cases, and makes sure that scripts with set -u won't fail. It might not be technically needed, but ensures we have the exact same behavior for all image types. The only somewhat difference is for emscripten images, but that's because we really can't have a viable prefix for them, so we just use em (so emcc and em++ work, but this is very different than allowing emgcc and emg++ to work, which don't exist).

[Emilgardis]

bors r+

[bors]

Build succeeded:

• conclusion