Add support for statically-linked musl libc Linux targets (-unknown-linux-musl)

[briansmith]

[gnzlbg]

Why is removing the libc crate dependency a goal ?

[cbiffle]

FYI - from updates on rust-lang/libc#659, getrandom is now available in libc on most Linux platforms. The holdout is not musl, but android/Bionic. I suggest decoupling the two parts of this issue an moving the getrandom portion into a separate bug.

[briansmith]

More discussion in #935

[briansmith]

@aig787's PR #1067 added the -musl targets for Linux x86_64 and i686 to CI/CD.

Still to do:

• Enabling static-pie for musl rust-lang/rust#70740 indicates that we need to verify that we're doing the right thing w.r.t. PIE (position independent code), especially when compiling the C and assembly code.
• Add Aarch64 musl targets to CI/CD.
• Minimize the dependencies installed. Currently the i686 job installs "libc6-dev-i386" and "gcc-multilib", but neither of those should be needed for -musl. (No longer relevant.)