arm64: add POE signal support

arm64: add POE signal support

Add PKEY support to signals, by saving and restoring POR_EL0 from the stackframe.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20240822151113.1479789-20-joey.gouly@arm.com
Signed-off-by: Will Deacon <will@kernel.org>