xen: add non-executable stack note uniformly

Currently the -Wa,--noexecstack parameter is only passed to the compiler
when building assembly files, as the assembler doesn't add such note for
stack attributes.

However, the default addition of the .note.GNU-stack section is dependent
on the ELF target used.  Adjust the passing of -Wa,--noexecstack so it's
added to CFLAGS instead of AFLAGS, and hence such section is also added to
object files generated from .c sources.  This fixes the complains from the
linker about missing .note.GNU-stack section when building x86-64 Xen on
Darwin platforms.

Note we could instead pass `-z noexecstack` to the linker, but that however
would mask any possible errors or unintended mismatches from previous
steps.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/Makefile

@@ -413,7 +413,7 @@ ifneq ($(CONFIG_CC_IS_CLANG),y)
 CFLAGS += -Wa,--strip-local-absolute
 endif
 
-$(call cc-option-add,AFLAGS,CC,-Wa$$(comma)--noexecstack)
+$(call cc-option-add,CFLAGS,CC,-Wa$$(comma)--noexecstack)
 
 LDFLAGS-$(call ld-option,--warn-rwx-segments) += --no-warn-rwx-segments