x86/boot: Prep work for 32bit object changes

Broken out of the subsequent patch for clarity.

 * Rename head-bin-objs to obj32
 * Use a .32.o suffix to distinguish these objects
 * Factor out $(LD32)

No functional change.

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

xen/arch/x86/boot/Makefile

@@ -1,15 +1,16 @@
 obj-bin-y += head.o
 
-head-bin-objs := cmdline.o reloc.o
+obj32 := cmdline.32.o
+obj32 += reloc.32.o
 
-nocov-y   += $(head-bin-objs)
-noubsan-y += $(head-bin-objs)
-targets   += $(head-bin-objs)
+nocov-y   += $(obj32)
+noubsan-y += $(obj32)
+targets   += $(obj32)
 
-head-bin-objs := $(addprefix $(obj)/,$(head-bin-objs))
+obj32 := $(addprefix $(obj)/,$(obj32))
 
 $(obj)/head.o: AFLAGS-y += -Wa$(comma)-I$(obj)
-$(obj)/head.o: $(head-bin-objs:.o=.bin)
+$(obj)/head.o: $(obj32:.32.o=.bin)
 
 CFLAGS_x86_32 := $(subst -m64,-m32 -march=i686,$(XEN_TREEWIDE_CFLAGS))
 $(call cc-options-add,CFLAGS_x86_32,CC,$(EMBEDDED_EXTRA_CFLAGS))
@@ -18,16 +19,20 @@ CFLAGS_x86_32 += -nostdinc -include $(filter %/include/xen/config.h,$(XEN_CFLAGS
 CFLAGS_x86_32 += $(filter -I% -O%,$(XEN_CFLAGS)) -D__XEN__
 
 # override for 32bit binaries
-$(head-bin-objs): CFLAGS_stack_boundary :=
-$(head-bin-objs): XEN_CFLAGS := $(CFLAGS_x86_32) -fpic
+$(obj32): CFLAGS_stack_boundary :=
+$(obj32): XEN_CFLAGS := $(CFLAGS_x86_32) -fpic
+
+$(obj)/%.32.o: $(src)/%.c FORCE
+	$(call if_changed_rule,cc_o_c)
 
 LDFLAGS_DIRECT-$(call ld-option,--warn-rwx-segments) := --no-warn-rwx-segments
 LDFLAGS_DIRECT += $(LDFLAGS_DIRECT-y)
+LD32 := $(LD) $(subst x86_64,i386,$(LDFLAGS_DIRECT))
 
 %.bin: %.lnk
 	$(OBJCOPY) -j .text -O binary $< $@
 
-%.lnk: %.o $(src)/build32.lds
-	$(LD) $(subst x86_64,i386,$(LDFLAGS_DIRECT)) -N -T $(filter %.lds,$^) -o $@ $<
+%.lnk: %.32.o $(src)/build32.lds
+	$(LD32) -N -T $(filter %.lds,$^) -o $@ $<
 
 clean-files := *.lnk *.bin