compiler: use builtin memset for non-pointer memclr

compiler: use builtin memset for non-pointer memclr

    For zeroing a range of memory that doesn't contain pointer, we
    can use builtin memset directly.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/184438

	* go-gcc.cc (Gcc_backend::Gcc_backend): Define __builtin_memset.

From-SVN: r272944