xen/build: pass -fzero-init-padding-bits=all to gcc15

See the respective bullet point in the Caveats section of
https://gcc.gnu.org/gcc-15/changes.html.

While I'm unaware of us currently relying on the pre-gcc15 behavior,
let's still play safe and retain what unknowingly we may have been
relying upon.

According to my observations, on x86 generated code changes
- somewhere deep in modify_bars(), presumably from the struct map_data
  initializer in apply_map() (a new MOVQ),
- in vpci_process_pending(), apparently again from the struct map_data
  initializer (and again a new MOVQ),
- near the top of find_cpio_data(), presumably from the struct cpio_data
  initializer (a MOVW changing to a MOVQ).

Requested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/Makefile

@@ -396,6 +396,7 @@ endif
 CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections -fdata-sections
 
 CFLAGS += -nostdinc -fno-builtin -fno-common
+$(call cc-option-add,CFLAGS,CC,-fzero-init-padding-bits=all)
 CFLAGS += -Werror -Wredundant-decls -Wwrite-strings -Wno-pointer-arith
 CFLAGS += -Wdeclaration-after-statement -Wuninitialized
 $(call cc-option-add,CFLAGS,CC,-Wvla)