base: Fix build errors with gcc 10.x

base: Fix build errors with gcc 10.x

This fixes conditions that perform a redundant check to
see if an unsigned value is greater than or equal to
zero. With gcc 10.x, this generates the following error
because of implicit usage of the "-Werror=type-limits"
flag.

"comparison of unsigned expression in '>= 0' is always true"

Change-Id: Ib1a88035ef5fba410d18de0adf614db4bc634faf
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30474
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>