util: Make the googletest library available to the m5 utility.

util: Make the googletest library available to the m5 utility.

The library will be available for the abis so that they can test
their unique call mechanisms, and also the main/native environment for
testing shared components.

Build instructions for things that should be built natively, ie unit
tests for common components, should go in the new SConscript.native.

Change-Id: I4a84b2cf2165c92dfb1b6d903b18b45e4cba1352
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27559
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>