selftests/intel_pstate: 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/intel_pstate/Makefile

@@ -2,10 +2,10 @@
 CFLAGS := $(CFLAGS) -Wall -D_GNU_SOURCE
 LDLIBS += -lm
 
-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 (x86,$(ARCH))
+ifeq (x86,$(ARCH_PROCESSED))
 TEST_GEN_FILES := msr aperf
 endif