powerpc: Fix __WARN_FLAGS() for use with Objtool

Commit 1e688dd ("powerpc/bug: Provide better flexibility to
WARN_ON/__WARN_FLAGS() with asm goto") updated __WARN_FLAGS() to use asm
goto, and added a call to 'unreachable()' after the asm goto for optimal
code generation. With CONFIG_OBJTOOL enabled, 'annotate_unreachable()'
statement in 'unreachable()' tries to note down the location of the
subsequent instruction in a separate elf section to aid code flow
analysis. However, on powerpc, this results in gcc emitting a call to a
symbol of size 0. This results in objtool complaining of "unannotated
intra-function call" since the target symbol is not a valid function
call destination.

Objtool wants this annotation for code flow analysis, which we are not
yet enabling on powerpc. As such, expand the call to 'unreachable()' in
__WARN_FLAGS() without annotate_unreachable():
        barrier_before_unreachable();
        __builtin_unreachable();

This still results in optimal code generation for __WARN_FLAGS(), while
getting rid of the objtool warning.

We still need barrier_before_unreachable() to work around gcc bugs 82365
and 106751:
- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106751

Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221114175754.1131267-2-sv@linux.ibm.com

arch/powerpc/include/asm/bug.h

@@ -99,7 +99,8 @@
 	__label__ __label_warn_on;				\
 								\
 	WARN_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags), __label_warn_on); \
-	unreachable();						\
+	barrier_before_unreachable();				\
+	__builtin_unreachable();				\
 								\
 __label_warn_on:						\
 	break;							\