arm64: set POR_EL0 for kernel threads

Restrict kernel threads to only have RWX overlays for pkey 0.  This matches
what arch/x86 does, by defaulting to a restrictive PKRU.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Kevin Brodsky <Kevin.Brodsky@arm.com>
Link: https://lore.kernel.org/r/20241001133618.1547996-2-joey.gouly@arm.com


Signed-off-by: Will Deacon <will@kernel.org>