powerpc/xmon: Relax frame size for clang

[ Upstream commit 9c87156cce5a63735d1218f0096a65c50a7a32aa ]

When building with clang (8 trunk, 7.0 release) the frame size limit is
hit:

 arch/powerpc/xmon/xmon.c:452:12: warning: stack frame size of 2576
 bytes in function 'xmon_core' [-Wframe-larger-than=]

Some investigation by Naveen indicates this is due to clang saving the
addresses to printf format strings on the stack.

While this issue is investigated, bump up the frame size limit for xmon
when building with clang.

Link: ClangBuiltLinux/linux#252
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/powerpc/xmon/Makefile

@@ -13,6 +13,12 @@ UBSAN_SANITIZE := n
 ORIG_CFLAGS := $(KBUILD_CFLAGS)
 KBUILD_CFLAGS = $(subst -mno-sched-epilog,,$(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS)))
 
+ifdef CONFIG_CC_IS_CLANG
+# clang stores addresses on the stack causing the frame size to blow
+# out. See https://github.com/ClangBuiltLinux/linux/issues/252
+KBUILD_CFLAGS += -Wframe-larger-than=4096
+endif
+
 ccflags-$(CONFIG_PPC64) := $(NO_MINIMAL_TOC)
 
 obj-y			+= xmon.o nonstdio.o spr_access.o