arm64: Do not export the compat-specific definitions to the user

This patch adds #ifdef __KERNEL__ guards around the COMPAT_* definitions
to avoid exporting them to user. AArch32 user requiring the kernel
headers must use those generated with ARCH=arm.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>