x86/boot: Align mbi2.c stack to 16 bytes

Most of Xen is built with a stack alignment of 8 bytes, but the UEFI spec
mandates 16 and UEFI services will fault if the stack is misaligned.

While the caller of efi_multiboot2_prelude() takes care to align the stack,
mbi2.c accidentally got the Xen-wide default of 8, and has a 50% chance of
crashing depending on how many variables the compiler decided to spill to the
stack.

Compile mbi2.c with the appropriate alignment for UEFI functionality.

Also take the opportunity to make it a fully .init object.

Fixes: eb21ce14d7 ('x86/boot: Rewrite EFI/MBI2 code partly in C')
Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
[rewrite the commit message]
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/efi/Makefile

@@ -9,11 +9,11 @@ $(obj)/%.o: $(src)/%.ihex FORCE
 $(obj)/boot.init.o: $(obj)/buildid.o
 
 $(call cc-option-add,cflags-stack-boundary,CC,-mpreferred-stack-boundary=4)
-$(addprefix $(obj)/,$(EFIOBJ-y)): CFLAGS_stack_boundary := $(cflags-stack-boundary)
+$(addprefix $(obj)/,$(EFIOBJ-y) mbi2.init.o): CFLAGS_stack_boundary := $(cflags-stack-boundary)
 
 obj-y := common-stub.o stub.o
 obj-$(XEN_BUILD_EFI) := $(filter-out %.init.o,$(EFIOBJ-y))
 obj-bin-$(XEN_BUILD_EFI) := $(filter %.init.o,$(EFIOBJ-y))
-obj-bin-y += mbi2.o
+obj-bin-y += mbi2.init.o
 extra-$(XEN_BUILD_EFI) += buildid.o relocs-dummy.o
 nocov-$(XEN_BUILD_EFI) += stub.o