Re-add aarch64 stuff removed by previous commit

Previous commit

dcff1547
"libc: changes to ppc64le musl branch to support building of rust on Alpine"

has removed stuff from mod.rs and added it to some submodules, but
missed the aarch64 submodule.

This copies the stuff that that commit added to the x86_64.rs submodule
and puts it into aarch64.rs.