util: Use the std namespace for memcpy and memset in the m5 util.

util: Use the std namespace for memcpy and memset in the m5 util.

The C++ versions of those functions are technically in the std
namespace, and while they may work without it (the C version leaking
through from somewhere?), they should still use it.

Change-Id: Ib53a7f0c41d7f5776221e7661c616ea51e8ed528
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31834
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>