EFI: fix tool chain capabilities detection

Commit f5a54e92 ("xen: move some arch CFLAGS into the common Rules.mk")
transformed CFLAGS assignments to CFLAGS-y ones, which collides with
the was xen/arch/x86/efi/Makefile determines whether the tol chain is
usable for an EFI build. Transform the block back to using CFLAGS.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -41,13 +41,12 @@ ALL_OBJS-y               += $(BASEDIR)/x
 ALL_OBJS-y               += $(BASEDIR)/arch/$(TARGET_ARCH)/built_in.o
 ALL_OBJS-$(x86)          += $(BASEDIR)/crypto/built_in.o
 
-CFLAGS-y                += -fno-builtin -fno-common
-CFLAGS-y                += -Werror -Wredundant-decls -Wno-pointer-arith
-CFLAGS-y                += -pipe
-CFLAGS-y                += -g -D__XEN__ -include $(BASEDIR)/include/xen/config.h
+CFLAGS += -fno-builtin -fno-common
+CFLAGS += -Werror -Wredundant-decls -Wno-pointer-arith
+CFLAGS += -pipe -g -D__XEN__ -include $(BASEDIR)/include/xen/config.h
 # Solaris puts stdarg.h &c in the system include directory.
 ifneq ($(XEN_OS),SunOS)
-CFLAGS-y                += -nostdinc -iwithprefix include
+CFLAGS += -nostdinc -iwithprefix include
 endif
 
 CFLAGS-$(XSM_ENABLE)    += -DXSM_ENABLE