KVM: arm64: Macros for setting/clearing FGT bits

There's a lot of boilerplate code for setting and clearing FGT
bits when activating guest traps. Refactor it into macros. These
macros will also be used in future patch series.

No functional change intended.

Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231214100158.2305400-15-tabba@google.com