arch-x86: add unconditional tag to calls/returns

arch-x86: add unconditional tag to calls/returns

The branch predictor checks whether an instruction is unconditional
branch before adding it or checking the RAS. With this change, the
RAS is significantly more effective for short running x86 workloads.

Change-Id: I60af5f2f583b898ad77f79f4b0478d6cda88fc21
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19448
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>