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 <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29392
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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