learning-gem5,tests: Fix 'valid_isas' in learning gem5 part 3
learning-gem5,tests: Fix 'valid_isas' in learning gem5 part 3

This was set to 'gcn3_x86_tag'. We only do this for quick X86 tests (to
minimize the build targets during the quick tests). This is not
necessary for the long tests.

Change-Id: I3bd20b53e04d129462e6f62a24f34b093f4a1b80
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59011
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5