tests: Changed 'long' boot tests to X86 from GCN3_X86

tests: Changed 'long' boot tests to X86 from GCN3_X86

We compile GCN3_X86 for the 'quick' tests, as a substitute for X86. We
compile X86 as part of our nightly tests, along with the running of the
'long' tests. This leads to a needless duplicate compilation of the X86
isa during our nightly tests. Therefore, this commit removes GCN3_X86
for the 'long' tests (only the x86 boot tests are affected).

Change-Id: Ifd8aaf0e7b8178c588ace33b27671d4ba9b353ed
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40415
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>