arm64: Work around broken .inst when defective gas is detected

.inst being largely broken with older binutils, it'd be better not
to emit it altogether when detecting such configuration (as it
leads to all kind of horrors when using alternatives).

Generalize the __emit_inst macro and use it extensively in
asm/sysreg.h, and make it generate a .long when a broken gas is
detected. The disassembly will be crap, but at least we can write
semi-sane code.

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