selftests: vm: enable cross-compilation

Selftests vm builds break when doing cross-compilation.  The Makefile
MACHINE variable incorrectly picks upp the host machine architecture.

If the CROSS_COMPILE variable is set, dig out the target host architecture
from CROSS_COMPILE, instead of calling uname.

Link: https://lkml.kernel.org/r/20230109114251.3349638-1-bjorn@kernel.orgSigned-off-by: Björn Töpel <bjorn@rivosinc.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/Makefile

@@ -5,7 +5,11 @@ LOCAL_HDRS += $(selfdir)/mm/local_config.h $(top_srcdir)/mm/gup_test.h
 
 include local_config.mk
 
+ifeq ($(CROSS_COMPILE),)
 uname_M := $(shell uname -m 2>/dev/null || echo not)
+else
+uname_M := $(shell echo $(CROSS_COMPILE) | grep -o '^[a-z0-9]\+')
+endif
 MACHINE ?= $(shell echo $(uname_M) | sed -e 's/aarch64.*/arm64/' -e 's/ppc64.*/ppc64/')
 
 # Without this, failed build products remain, with up-to-date timestamps,