selftests: vm: Do not override definition of ARCH

Independent builds of the vm selftests is currently broken because
commit 7549b336 ("selftests: vm: Build/Run 64bit tests only on
64bit arch") overrides the value of ARCH with the machine name from
uname. This does not always match the architecture names used for
tasks like header installation.

E.g. for building tests on powerpc64, we need ARCH=powerpc
and not ARCH=ppc64 or ARCH=ppc64le. Otherwise, the build
fails as shown below.

  $ uname -m
  ppc64le

  $ make -C tools/testing/selftests/vm
  make: Entering directory '/home/sandipan/linux/tools/testing/selftests/vm'
  make --no-builtin-rules ARCH=ppc64le -C ../../../.. headers_install
  make[1]: Entering directory '/home/sandipan/linux'
  Makefile:653: arch/ppc64le/Makefile: No such file or directory
  make[1]: *** No rule to make target 'arch/ppc64le/Makefile'.  Stop.
  make[1]: Leaving directory '/home/sandipan/linux'
  ../lib.mk:50: recipe for target 'khdr' failed
  make: *** [khdr] Error 2
  make: Leaving directory '/home/sandipan/linux/tools/testing/selftests/vm'

Fixes: 7549b336 ("selftests: vm: Build/Run 64bit tests only on 64bit arch")
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>