arm64: support keyctl() system call in 32-bit mode

commit 5c2a6259 upstream.

As is the case for a number of other architectures that have a 32-bit
compat mode, enable KEYS_COMPAT if both COMPAT and KEYS are enabled.
This allows AArch32 programs to use the keyctl() system call when
running on an AArch64 kernel.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>