powerpc: Add -mno-altivec for ARCH=powerpc builds

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/Makefile

@@ -93,6 +93,9 @@ else
 endif
 endif
 
+# No AltiVec instruction when building kernel
+CFLAGS += $(call cc-option,-mno-altivec)
+
 # Enable unit-at-a-time mode when possible. It shrinks the
 # kernel considerably.
 CFLAGS += $(call cc-option,-funit-at-a-time)