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 <[e-mail]>
Acked-by: Michael Ellerman <[e-mail]>
Cc: Pranith Kumar <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/membarrier/Makefile

···
 CFLAGS += -g -I../../../../usr/include/
 
-all:
-	$(CC) $(CFLAGS) membarrier_test.c -o membarrier_test
-
 TEST_PROGS := membarrier_test
+
+all: $(TEST_PROGS)
 
 include ../lib.mk
 
 clean:
-	$(RM) membarrier_test
+	$(RM) $(TEST_PROGS)

tools/testing/selftests/membarrier/membarrier_test.c

···
 #define _GNU_SOURCE
-#define __EXPORTED_HEADERS__
-
 #include <linux/membarrier.h>
-#include <asm-generic/unistd.h>
-#include <sys/syscall.h>
+#include <syscall.h>
 #include <stdio.h>
 #include <errno.h>
 #include <string.h>