arm64: insn: Add BUILD_BUG_ON() for invalid masks

Detect invalid instruction masks at build time. Some versions of GCC can
warn about the situation, but not all of them, it seems.

Suggested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>