Use musl-gcc, fix musl headers

ci/run-travis.sh

libc-test/build.rs

src/unix/bsd/mod.rs

src/unix/mod.rs

src/unix/notbsd/android/mod.rs

src/unix/notbsd/linux/mod.rs

src/unix/notbsd/linux/notmips/mod.rs

src/unix/notbsd/mod.rs