FROMLIST: arm64: avoid double ISB on kernel entry

Although an ISB is required in order to make the MTE-related system
register update to GCR_EL1 effective, and the same is true for
PAC-related updates to SCTLR_EL1 or APIAKey{Hi,Lo}_EL1, we issue two
ISBs on machines that support both features while we only need to
issue one. To avoid the unnecessary additional ISB, remove the ISBs
from the PAC and MTE-specific alternative blocks and add a couple
of additional blocks that cause us to only execute one ISB if both
features are supported.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/Idee7e8114d5ae5a0b171d06220a0eb4bb015a51c
Change-Id: Idee7e8114d5ae5a0b171d06220a0eb4bb015a51c
Bug: 192536783
Link: https://lore.kernel.org/linux-arm-kernel/20210714013631.3994485-1-pcc@google.com/T/