x86/boot: Add missing __XEN__ definition for 32 bit code

We are compiling Xen source code so we should define __XEN__ macro.
We don't want to import all definitions from XEN_CFLAGS (as done for
other options) because most of them are processor dependent and
do not apply to 32 bit.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/boot/Makefile

@@ -15,7 +15,7 @@ CFLAGS_x86_32 := $(subst -m64,-m32 -march=i686,$(XEN_TREEWIDE_CFLAGS))
 $(call cc-options-add,CFLAGS_x86_32,CC,$(EMBEDDED_EXTRA_CFLAGS))
 CFLAGS_x86_32 += -Werror -fno-builtin -g0 -msoft-float -mregparm=3
 CFLAGS_x86_32 += -nostdinc -include $(filter %/include/xen/config.h,$(XEN_CFLAGS))
-CFLAGS_x86_32 += $(filter -I% -O%,$(XEN_CFLAGS))
+CFLAGS_x86_32 += $(filter -I% -O%,$(XEN_CFLAGS)) -D__XEN__
 
 # override for 32bit binaries
 $(head-bin-objs): CFLAGS_stack_boundary :=