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>

arch/x86/include/asm/asm.h

@@ -120,6 +120,8 @@
 # define CC_OUT(c) [_cc_ ## c] "=qm"
 #endif
 
+#ifdef __KERNEL__
+
 /* Exception table entry */
 #ifdef __ASSEMBLY__
 # define _ASM_EXTABLE_HANDLE(from, to, handler)			\
@@ -186,4 +188,6 @@ register unsigned long current_stack_pointer asm(_ASM_SP);
 #define ASM_CALL_CONSTRAINT "+r" (current_stack_pointer)
 #endif /* __ASSEMBLY__ */
 
+#endif /* __KERNEL__ */
+
 #endif /* _ASM_X86_ASM_H */

tools/arch/x86/include/asm/asm.h

@@ -120,6 +120,8 @@
 # define CC_OUT(c) [_cc_ ## c] "=qm"
 #endif
 
+#ifdef __KERNEL__
+
 /* Exception table entry */
 #ifdef __ASSEMBLY__
 # define _ASM_EXTABLE_HANDLE(from, to, handler)			\
@@ -186,4 +188,6 @@ register unsigned long current_stack_pointer asm(_ASM_SP);
 #define ASM_CALL_CONSTRAINT "+r" (current_stack_pointer)
 #endif /* __ASSEMBLY__ */
 
+#endif /* __KERNEL__ */
+
 #endif /* _ASM_X86_ASM_H */