x86/asm: Make <asm/asm.h> valid on cross-builds as well

Stephen Rothwell reported that the objtool cross-build breaks on
non-x86 hosts:

  > tools/arch/x86/include/asm/asm.h:185:24: error: invalid register name for 'current_stack_pointer'
  >   185 | register unsigned long current_stack_pointer asm(_ASM_SP);
  >       |                        ^~~~~~~~~~~~~~~~~~~~~

The PowerPC host obviously doesn't know much about x86 register names.

Protect the kernel-specific bits of <asm/asm.h>, so that it can be
included by tooling and cross-built.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>