build: fix exported variable name CFLAGS_stack_boundary

Exporting a variable with a dash doesn't work reliably, they may be
striped from the environment when calling a sub-make or sub-shell.

CFLAGS-stack-boundary start to be removed from env in patch "build:
set ALL_OBJS in main Makefile; move prelink.o to main Makefile" when
running `make "ALL_OBJS=.."` due to the addition of the quote. At
least in my empirical tests.

Fixes: 2740d96efd ("xen/build: have the root Makefile generates the CFLAGS")
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/Rules.mk

@@ -6,5 +6,5 @@ object_label_flags = '-D__OBJECT_LABEL__=$(subst $(BASEDIR)/,,$(CURDIR))/$@'
 else
 object_label_flags = '-D__OBJECT_LABEL__=$(subst /,$$,$(subst -,_,$(subst $(BASEDIR)/,,$(CURDIR))/$@))'
 endif
-c_flags += $(object_label_flags) $(CFLAGS-stack-boundary)
-a_flags += $(object_label_flags) $(CFLAGS-stack-boundary)
+c_flags += $(object_label_flags) $(CFLAGS_stack_boundary)
+a_flags += $(object_label_flags) $(CFLAGS_stack_boundary)

xen/arch/x86/arch.mk

@@ -49,8 +49,8 @@ CFLAGS-$(CONFIG_INDIRECT_THUNK) += -fno-jump-tables
 
 # If supported by the compiler, reduce stack alignment to 8 bytes. But allow
 # this to be overridden elsewhere.
-$(call cc-option-add,CFLAGS-stack-boundary,CC,-mpreferred-stack-boundary=3)
-export CFLAGS-stack-boundary
+$(call cc-option-add,CFLAGS_stack_boundary,CC,-mpreferred-stack-boundary=3)
+export CFLAGS_stack_boundary
 
 ifeq ($(CONFIG_UBSAN),y)
 # Don't enable alignment sanitisation.  x86 has efficient unaligned accesses,

xen/arch/x86/efi/Makefile

@@ -12,7 +12,7 @@ EFIOBJ-y := boot.init.o pe.init.o ebmalloc.o runtime.o
 EFIOBJ-$(CONFIG_COMPAT) += compat.o
 
 $(call cc-option-add,cflags-stack-boundary,CC,-mpreferred-stack-boundary=4)
-$(EFIOBJ-y): CFLAGS-stack-boundary := $(cflags-stack-boundary)
+$(EFIOBJ-y): CFLAGS_stack_boundary := $(cflags-stack-boundary)
 
 obj-y := stub.o
 obj-$(XEN_BUILD_EFI) := $(filter-out %.init.o,$(EFIOBJ-y))