fastmodel: Switch the diagnostic pragmas to GCC from clang.

fastmodel: Switch the diagnostic pragmas to GCC from clang.

Clang can handle both, and GCC throws a fit if it sees pragmas for
clang.

Change-Id: Ie9f2789f45706223b11ed5acdf8b371de6e7ee24
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23321
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>