x86: don't have gcc over-align data

For (aiui) backwards compatibility reasons, gcc defaults to a mode that
was the exclusive one up to gcc4.8, establishing 16- or even 32-byte
alignment for aggregates larger than a certain size. We don't rely on
such, and hence we can do with the psABI-compliant 16-byte alignment.

Savings in the build I'm looking at:
- .data.ro_after_init		 344 bytes
- .rodata + .data.rel.ro	1904 bytes
- .init.*data.cf_clobber	 232 bytes
- .init (overall)		 688 bytes
- .data.read_mostly		 864 bytes
- .data				 600 bytes
- .bss				1472 bytes

Overall xen-syms' _end happens to move down there by 2 pages.

Clang doesn't support the option, presumably because they never over-
aligned data.

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

xen/arch/x86/arch.mk

@@ -8,6 +8,9 @@ CFLAGS += -DXEN_IMG_OFFSET=$(XEN_IMG_OFFSET)
 # Prevent floating-point variables from creeping into Xen.
 CFLAGS += -msoft-float
 
+# Don't needlessly over-align larger aggregates.
+CFLAGS-$(CONFIG_CC_IS_GCC) += -malign-data=abi
+
 $(call cc-options-add,CFLAGS,CC,$(EMBEDDED_EXTRA_CFLAGS))
 $(call cc-option-add,CFLAGS,CC,-Wnested-externs)
 $(call as-option-add,CFLAGS,CC,".equ \"x\"$(comma)1",-DHAVE_AS_QUOTED_SYM)