powerpc: Disable -Wbuiltin-requires-header when setjmp is used

The powerpc kernel uses setjmp which causes a warning when building
with clang:

  In file included from arch/powerpc/xmon/xmon.c:51:
  ./arch/powerpc/include/asm/setjmp.h:15:13: error: declaration of
  built-in function 'setjmp' requires inclusion of the header <setjmp.h>
        [-Werror,-Wbuiltin-requires-header]
  extern long setjmp(long *);
              ^
  ./arch/powerpc/include/asm/setjmp.h:16:13: error: declaration of
  built-in function 'longjmp' requires inclusion of the header <setjmp.h>
        [-Werror,-Wbuiltin-requires-header]
  extern void longjmp(long *, long);
              ^

This *is* the header and we're not using the built-in setjump but
rather the one in arch/powerpc/kernel/misc.S. As the compiler warning
does not make sense, it for the files where setjmp is used.
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
[mpe: Move subdir-ccflags in xmon/Makefile to not clobber -Werror]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/Makefile

@@ -5,6 +5,9 @@
 
 CFLAGS_ptrace.o		+= -DUTS_MACHINE='"$(UTS_MACHINE)"'
 
+# Disable clang warning for using setjmp without setjmp.h header
+CFLAGS_crash.o		+= $(call cc-disable-warning, builtin-requires-header)
+
 subdir-ccflags-$(CONFIG_PPC_WERROR) := -Werror
 
 ifdef CONFIG_PPC64

arch/powerpc/xmon/Makefile

 # SPDX-License-Identifier: GPL-2.0
 # Makefile for xmon
 
-subdir-ccflags-$(CONFIG_PPC_WERROR) := -Werror
+# Disable clang warning for using setjmp without setjmp.h header
+subdir-ccflags-y := $(call cc-disable-warning, builtin-requires-header)
+
+subdir-ccflags-$(CONFIG_PPC_WERROR) += -Werror
 
 GCOV_PROFILE := n
 UBSAN_SANITIZE := n