arm64/sme: Manually encode SME instructions

As with SVE rather than impose ambitious toolchain requirements for SME
we manually encode the few instructions which we require in order to
perform the work the kernel needs to do. The instructions used to save
and restore context are provided as assembler macros while those for
entering and leaving streaming mode are done in asm volatile blocks
since they are expected to be used from C.

We could do the SMSTART and SMSTOP operations with read/modify/write
cycles on SVCR but using the aliases provided for individual field
accesses should be slightly faster. These instructions are aliases for
MSR but since our minimum toolchain requirements are old enough to mean
that we can't use the sX_X_cX_cX_X form and they always use xzr rather
than taking a value like write_sysreg_s() wants we just use .inst.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220419112247.711548-7-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>