arm: fix build with gcc 4.7.x

As was already pointed out months ago (see
http://lists.xen.org/archives/html/xen-devel/2012-02/msg00826.html),
gcc 4.7.x (imo validly) refuses to take both -mcpu=cortex-a15 and
-march=armv7-a due to conflicting feature sets causing amibiguity in
instruction selection. Since the former implies the latter, just use
the former (and drop the -march=).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/arm/Rules.mk
+++ b/xen/arch/arm/Rules.mk
@@ -24,7 +24,7 @@ ifneq ($(call cc-option,$(CC),-fvisibili
 CFLAGS += -DGCC_HAS_VISIBILITY_ATTRIBUTE
 endif
 
-CFLAGS += -march=armv7-a -mcpu=cortex-a15 -mfpu=vfpv3 -mfloat-abi=softfp
+CFLAGS += -mcpu=cortex-a15 -mfpu=vfpv3 -mfloat-abi=softfp
 
 # Require GCC v3.4+ (to avoid issues with alignment constraints in Xen headers)
 check-$(gcc) = $(call cc-ver-check,CC,0x030400,"Xen requires at least gcc-3.4")