Reorganize constant declarations to make up space for MIPS64-specific ones

src/unix/linux_like/linux/musl/b64/aarch64.rs

src/unix/linux_like/linux/musl/b64/mod.rs

src/unix/linux_like/linux/musl/b64/powerpc64.rs

src/unix/linux_like/linux/musl/b64/x86_64.rs