Link to libgcc for `armv5te-unknown-linux-musleabi` target.

[Alexhuszagh]

Fixes missing sync builtins required for libraries such as OpenSSL, specifically, the __sync_X_and_fetch builtins.

Closes #367. Improves documentation related to #906.

[Emilgardis]

I'm not sure this is something we should do since it's an upstream issue and will eventually be fixed hopefully.

[Alexhuszagh]

I'm not sure this is something we should do since it's an upstream issue and will eventually be fixed hopefully.

It is, it's already been patched (I submitted https://github.com/rust-lang/compiler-builtins/pull/484 which was merged), but we need to wait for a stable release similar to what we do in aarch64-linux-musl-gcc.sh Similarly, we can remove -libgcc shortly when https://github.com/rust-lang/compiler-builtins/pull/483 makes it to stable.

[Alexhuszagh]

bors r=Emilgardis

[bors[bot]]

Build failed:

• conclusion

[Alexhuszagh]

bors try --target mips64-unknown-linux-muslabi64

[bors[bot]]

try

Build succeeded:

• conclusion

[Alexhuszagh]

bors r=Emilgardis

[bors[bot]]

Build succeeded:

• conclusion