guix: pass `--with-newlib` to gcc-sans-libc build

When building our initial libc-less gcc, we can pass --with-newlib, which ensures inhibit_libc is defined, and better indicates our intent.

This is somewhat related to, but doesn't solve #22458 (closed).

Co-authored-by: Carl Dong contact@carldong.me Co-authored-by: Hennadii Stepanov 32963518+hebasto@users.noreply.github.com