EFI: fix tool chain capabilities detection

EFI: fix tool chain capabilities detection

Commit f5a54e92 ("xen: move some arch CFLAGS into the common Rules.mk")
transformed CFLAGS assignments to CFLAGS-y ones, which collides with
the was xen/arch/x86/efi/Makefile determines whether the tol chain is
usable for an EFI build. Transform the block back to using CFLAGS.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Acked-by: Keir Fraser <keir@xen.org>