runtime: use builtin memmove directly

runtime: use builtin memmove directly

    We can use the intrinsic memmove directly, without going through
    C.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/170004

	* go-gcc.cc (Gcc_backend::Gcc_backend): Define memmove builtin.

From-SVN: r271016