`rustup target add aarch64-unknown-none-softfloat` is now a thing.
It is no longer required to build rust-std on-demand with `xbuild`.
This reduces compile-times significantly.
In rust-lang/rust@679657b863 a breaking
change was introduced that now requires compiler_builtins. Add them,
and use the opportunity to kick out rlibc, since memset() et al. can
be provided by compiler intrinsics.