Link to libgcc when statically linking musl

[Amanieu]

On some architectures (e.g. aarch64) musl depends on some libgcc functions (__addtf3, __multf3, __subtf3) for long double arithmetic that it uses internally. Unfortunately we don't provide these functions in compiler-builtins, so we instead need to get them from libgcc.

Fixes:
#46651
rust-lang/compiler-builtins#201
rust-lang/compiler-builtins#217

[rust-highfive]

r? @varkor

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[Amanieu]

r? @alexcrichton

[alexcrichton]

As is I think this may be a breaking change because we don't ship libgcc.a with the musl target but we do otherwise support compiling for musl with just a linker. That means that on systems that don't have their own libgcc.a (or an incompatible one) compiling for musl will fail I think?

[Amanieu]

Alternatively, I think we could force libgcc to be statically linked into the libc crate when it is built. This would solve the distribution issue.

[Amanieu]

Closing in favor of rust-lang/libc#1034.