powerpc: Avoid clang warnings around setjmp and longjmp

[ Upstream commit c9029ef9c95765e7b63c4d9aa780674447db1ec0 ]

Commit aea447141c7e ("powerpc: Disable -Wbuiltin-requires-header when
setjmp is used") disabled -Wbuiltin-requires-header because of a
warning about the setjmp and longjmp declarations.

r367387 in clang added another diagnostic around this, complaining
that there is no jmp_buf declaration.

  In file included from ../arch/powerpc/xmon/xmon.c:47:
  ../arch/powerpc/include/asm/setjmp.h:10:13: error: declaration of
  built-in function 'setjmp' requires the declaration of the 'jmp_buf'
  type, commonly provided in the header <setjmp.h>.
  [-Werror,-Wincomplete-setjmp-declaration]
  extern long setjmp(long *);
              ^
  ../arch/powerpc/include/asm/setjmp.h:11:13: error: declaration of
  built-in function 'longjmp' requires the declaration of the 'jmp_buf'
  type, commonly provided in the header <setjmp.h>.
  [-Werror,-Wincomplete-setjmp-declaration]
  extern void longjmp(long *, long);
              ^
  2 errors generated.

We are not using the standard library's longjmp/setjmp implementations
for obvious reasons; make this clear to clang by using -ffreestanding
on these files.

Cc: stable@vger.kernel.org # 4.14+
Suggested-by: NSegher Boessenkool <segher@kernel.crashing.org>
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NNathan Chancellor <natechancellor@gmail.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191119045712.39633-3-natechancellor@gmail.comSigned-off-by: NSasha Levin <sashal@kernel.org>

arch/powerpc/kernel/Makefile

@@ -5,8 +5,8 @@
 
 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)
+# Avoid clang warnings around longjmp/setjmp declarations
+CFLAGS_crash.o += -ffreestanding
 
 subdir-ccflags-$(CONFIG_PPC_WERROR) := -Werror
 

arch/powerpc/xmon/Makefile

 # SPDX-License-Identifier: GPL-2.0
 # Makefile for xmon
 
-# Disable clang warning for using setjmp without setjmp.h header
-subdir-ccflags-y := $(call cc-disable-warning, builtin-requires-header)
+# Avoid clang warnings around longjmp/setjmp declarations
+subdir-ccflags-y := -ffreestanding
 
 subdir-ccflags-$(CONFIG_PPC_WERROR) += -Werror