selftests: vm: pkeys: fix multilib builds for x86

This ensures that both 32-bit and 64-bit binaries are generated when this
is built on a x86_64 system.  Most of the changes have been borrowed from
tools/testing/selftests/x86/Makefile.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Cc: "Desnes A. Nunes do Rosario" <desnesn@linux.vnet.ibm.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Suchanek <msuchanek@suse.de>
Cc: Shuah Khan <shuah@kernel.org>
Link: http://lkml.kernel.org/r/0326a442214d7a1b970d38296e63df3b217f5912.1585646528.git.sandipan@linux.ibm.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>