powerpc: Avoid -maltivec when using clang integrated assembler

Check the assembler supports -maltivec by wrapping it with
call as-option.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Makefile

@@ -221,7 +221,7 @@ KBUILD_CFLAGS		+= -mno-sched-epilog
 endif
 
 cpu-as-$(CONFIG_4xx)		+= -Wa,-m405
-cpu-as-$(CONFIG_ALTIVEC)	+= -Wa,-maltivec
+cpu-as-$(CONFIG_ALTIVEC)	+= $(call as-option,-Wa$(comma)-maltivec)
 cpu-as-$(CONFIG_E200)		+= -Wa,-me200
 
 KBUILD_AFLAGS += $(cpu-as-y)