powerpc/boot: Set target when cross-compiling for clang

Clang needs to be told which target it is building for when cross
compiling.

Link: ClangBuiltLinux/linux#259
Signed-off-by: Joel Stanley <joel@jms.id.au>
Tested-by: Daniel Axtens <dja@axtens.net> # powerpc 64-bit BE
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/powerpc/boot/Makefile

@@ -55,6 +55,11 @@ BOOTAFLAGS	:= -D__ASSEMBLY__ $(BOOTCFLAGS) -traditional -nostdinc
 
 BOOTARFLAGS	:= -cr$(KBUILD_ARFLAGS)
 
+ifdef CONFIG_CC_IS_CLANG
+BOOTCFLAGS += $(CLANG_FLAGS)
+BOOTAFLAGS += $(CLANG_FLAGS)
+endif
+
 ifdef CONFIG_DEBUG_INFO
 BOOTCFLAGS	+= -g
 endif