selftests/kexec: fix build for ARCH=x86_64

Handle the scenario where the build is launched with the ARCH envvar
defined as x86_64.
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/kexec/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
 # Makefile for kexec tests
 
-uname_M := $(shell uname -m 2>/dev/null || echo not)
-ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
+ARCH ?= $(shell uname -m 2>/dev/null || echo not)
+ARCH_PROCESSED := $(shell echo $(ARCH) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
 
-ifeq ($(ARCH),$(filter $(ARCH),x86 ppc64le))
+ifeq ($(ARCH_PROCESSED),$(filter $(ARCH_PROCESSED),x86 ppc64le))
 TEST_PROGS := test_kexec_load.sh test_kexec_file_load.sh
 TEST_FILES := kexec_common_lib.sh