build: Use SUBARCH

Instead of doing additional `uname -m` - use provided $(SUBARCH) to detect
what architecture flavour the build should produce the result for.

Fixes two things:
- zdtm make now correctly supplies $(USERCFLAGS)
- subtly fixes cross compilation by providing a way to specify $(SUBARCH)

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>

Makefile

@@ -17,8 +17,6 @@ ifeq ($(origin HOSTCFLAGS), undefined)
         HOSTCFLAGS := $(CFLAGS) $(USERCFLAGS)
 endif
 
-UNAME-M := $(shell uname -m)
-
 #
 # Supported Architectures
 ifneq ($(filter-out x86 arm aarch64 ppc64 s390,$(ARCH)),)
@@ -27,14 +25,14 @@ endif
 
 # The PowerPC 64 bits architecture could be big or little endian.
 # They are handled in the same way.
-ifeq ($(UNAME-M),ppc64)
+ifeq ($(SUBARCH),ppc64)
         error := $(error ppc64 big endian is not yet supported)
 endif
 
 #
 # Architecture specific options.
 ifeq ($(ARCH),arm)
-        ARMV		:= $(shell echo $(UNAME-M) | sed -nr 's/armv([[:digit:]]).*/\1/p; t; i7')
+        ARMV		:= $(shell echo $(SUBARCH) | sed -nr 's/armv([[:digit:]]).*/\1/p; t; i7')
 
         ifeq ($(ARMV),6)
                 USERCFLAGS += -march=armv6

test/zdtm/Makefile.inc

@@ -1,7 +1,8 @@
 .SUFFIXES:
 MAKEFLAGS	+= -r
 
-ARCH ?= $(shell uname -m | sed          \
+SUBARCH ?= $(shell uname -m)
+ARCH	?= $(shell echo $(SUBARCH) | sed	\
                 -e s/i.86/x86/			\
                 -e s/x86_64/x86/		\
                 -e s/sun4u/sparc64/		\
@@ -19,7 +20,7 @@ ifeq ($(ARCH),arm64)
 endif
 
 ifeq ($(ARCH),arm)
-	ARMV := $(shell echo $(UNAME-M) | sed -nr 's/armv([[:digit:]]).*/\1/p; t; i7')
+	ARMV := $(shell echo $(SUBARCH) | sed -nr 's/armv([[:digit:]]).*/\1/p; t; i7')
 
 	ifeq ($(ARMV),6)
 		USERCFLAGS += -march=armv6