arm64: pt_regs: assert pt_regs is a multiple of 16 bytes

arm64: pt_regs: assert pt_regs is a multiple of 16 bytes

To ensure that the stack is correctly aligned when branching to C code,
we require that struct pt_regs is a multiple of 16 bytes, as noted in a
comment.

Add an explicit assertion for this, so that any accidental violation of
this requirement will be caught by the compiler.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Puranjay Mohan <puranjay12@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241017092538.1859841-2-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>