test/arm: Fix building on armv7l architecture

The issue here is that ARCH is not overwritten if the caller Makefile
already define it, and the upper Makefile is not overwriting the ARCH
define to "arm", when it is "armv7l" for instance, so the SRCARCH is
set to "armv7l" which is wrong.

With this patch, SRCARCH is using the inherinted define if called from
the top level Makefile.

This is required to build test on armv7l architecture (qemu).
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/Makefile.inc

@@ -8,7 +8,7 @@ ARCH ?= $(shell uname -m | sed		\
 		-e s/mips.*/mips/	\
 		-e s/sh[234].*/sh/)
 
-SRCARCH := $(ARCH)
+SRCARCH ?= $(ARCH)
 
 ifeq ($(ARCH),i386)
 	SRCARCH := x86