x86: Annotate call_on_stack()

vmlinux.o: warning: objtool: page_fault_oops()+0x13c: unreachable instruction

0000 000000000005b460 <page_fault_oops>:
...
0128    5b588:  49 89 23                mov    %rsp,(%r11)
012b    5b58b:  4c 89 dc                mov    %r11,%rsp
012e    5b58e:  4c 89 f2                mov    %r14,%rdx
0131    5b591:  48 89 ee                mov    %rbp,%rsi
0134    5b594:  4c 89 e7                mov    %r12,%rdi
0137    5b597:  e8 00 00 00 00          call   5b59c <page_fault_oops+0x13c>    5b598: R_X86_64_PLT32   handle_stack_overflow-0x4
013c    5b59c:  5c                      pop    %rsp

vmlinux.o: warning: objtool: sysvec_reboot()+0x6d: unreachable instruction

0000 00000000000033f0 <sysvec_reboot>:
...
005d     344d:  4c 89 dc                mov    %r11,%rsp
0060     3450:  e8 00 00 00 00          call   3455 <sysvec_reboot+0x65>        3451: R_X86_64_PLT32    irq_enter_rcu-0x4
0065     3455:  48 89 ef                mov    %rbp,%rdi
0068     3458:  e8 00 00 00 00          call   345d <sysvec_reboot+0x6d>        3459: R_X86_64_PC32     .text+0x47d0c
006d     345d:  e8 00 00 00 00          call   3462 <sysvec_reboot+0x72>        345e: R_X86_64_PLT32    irq_exit_rcu-0x4
0072     3462:  5c                      pop    %rsp

Both cases are due to a call_on_stack() calling a __noreturn function.
Since that's an inline asm, GCC can't do anything about the
instructions after the CALL. Therefore put in an explicit
ASM_REACHABLE annotation to make sure objtool and gcc are consistently
confused about control flow.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154319.468805622@infradead.org

arch/x86/include/asm/irq_stack.h

@@ -99,7 +99,8 @@
 }
 
 #define ASM_CALL_ARG0							\
-	"call %P[__func]				\n"
+	"call %P[__func]				\n"		\
+	ASM_REACHABLE
 
 #define ASM_CALL_ARG1							\
 	"movq	%[arg1], %%rdi				\n"		\