KVM: arm64: Specialize handling of host fpsimd state on trap

KVM: arm64: Specialize handling of host fpsimd state on trap

In subsequent patches, n/vhe will diverge on saving the host
fpsimd/sve state when taking a guest fpsimd/sve trap. Add a
specialized helper to handle it.

No functional change intended.

Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://lore.kernel.org/r/20240603122852.3923848-5-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>