FROMLIST: arm64: mte: optimize GCR_EL1 modification on kernel entry/exit

Accessing GCR_EL1 and issuing an ISB can be expensive on some
microarchitectures. Although we must write to GCR_EL1, we can
restructure the code to avoid reading from it because the new value
can be derived entirely from the exclusion mask, which is already in
a GPR. Do so.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I560a190a74176ca4cc5191dad08f77f6b1577c75
Change-Id: If73813daf8a24209b7582ae7b5e9a2a30004b086
Bug: 192536783
Link: https://lore.kernel.org/linux-arm-kernel/20210714013638.3995315-1-pcc@google.com/T/