arm64: compat: Allow signal page to be remapped

For compatability with 32-bit Arm, allow the compat signal page to be
remapped via mremap().

Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210318170738.7756-4-will@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>