selftests/powerpc: Add wrapper for gettid

The gettid() syscall wrapper was first introduced in
glibc 2.30. This adds a wrapper for use in distros
running older versions.
Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8ca3b0eeda989707815d1cf337cc33f090408965.1595821792.git.sandipan@linux.ibm.com

tools/testing/selftests/powerpc/include/utils.h

@@ -42,6 +42,16 @@ int perf_event_enable(int fd);
 int perf_event_disable(int fd);
 int perf_event_reset(int fd);
 
+#if !defined(__GLIBC_PREREQ) || !__GLIBC_PREREQ(2, 30)
+#include <unistd.h>
+#include <sys/syscall.h>
+
+static inline pid_t gettid(void)
+{
+	return syscall(SYS_gettid);
+}
+#endif
+
 static inline bool have_hwcap(unsigned long ftr)
 {
 	return ((unsigned long)get_auxv_entry(AT_HWCAP) & ftr) == ftr;