tools: selftests: fix build issue with make kselftests target

Fix the typo of ARCH when running 'make kselftests'.  Change the 'X86'
to 'x86'.  Test by compilation.

Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Shuah Khan <shuah.kh@samsung.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>