membarrier: clean up selftest

We don't need to specify an explicit rule in the Makefile, the implicit
one will do the same.  The "__EXPORTED_HEADERS__" define is not needed,
because we build the test against the installed kernel headers, not the
in-tree kernel headers.  Re-use "$(TEST_PROGS)" in the clean target
rather than spelling the executable name twice.  Include <unistd.h>
rather than the rather specific <asm-generic/unistd.h>.  Include
<syscall.h> rather than <sys/syscall.h>.  In both cases, the former
header is located in a standard location and includes the latter.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Pranith Kumar <bobby.prani@gmail.com>
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>