base: Stop "using namespace std".

base: Stop "using namespace std".

As the std namespace expands, it becomes more and more likely that
blanketly importing all its symbols will cause a collision. Also, when
it was imported, the std:: was used or left off arbitrarily, sometimes
inconsistently even in the same function signature.

Change-Id: Ie30cbab154b00c60433908a206c229230d2b109f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39536
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>