misc: Remove GCN3 as a build target

misc: Remove GCN3 as a build target

This target currently doesn't compile, so remove it from the list of
supported ISAs for the gem5-20 release. We can add this target back
after the compilation errors have been fixed.

Change-Id: I2b121824fcfee59b62d7d24600ddd0eece884c6b
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29392
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>