base: Add some error handling to compiler.hh.

base: Add some error handling to compiler.hh.

Rather than just leaving some macros undefined if none of the scenarios
we checked for match, we should report an error so it's clear what
happened. Otherwise the places the macros are used will just not compile
properly, or worse will silently not work correctly.

Change-Id: Ie010d6b6d1b6a1496a45d9ebc0d75d1c804df12f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35275
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>