tests,arch-gcn3,arch-x86: Changed X86 testlib tests to GCN3_X86

tests,arch-gcn3,arch-x86: Changed X86 testlib tests to GCN3_X86

In an effort to get better test coverage, we've changed all X86 tests to
use GCN3_X86. This will, as a minimum, ensure that GCN3 is regularly
compiled. GCN3_X86 is a superset of X86 and all X86 tests should pass on
GCN3_X86.

Change-Id: I2684edfc4e48c3e311a400231293a9e04c701130
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38279
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>