use glibc provided gettid API for glibc 2.30+

glibc 2.30 introduced this function see [1]
so it's best to detect it
and provide fallback only if it's not present

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=6399

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/src/core/lib/gpr/log_linux.cc
+++ b/src/core/lib/gpr/log_linux.cc
@@ -40,7 +40,13 @@
 #include <time.h>
 #include <unistd.h>
 
+#if  defined(__GLIBC__)
+#if !__GLIBC_PREREQ(2,29)
 static long gettid(void) { return syscall(__NR_gettid); }
+#endif
+#else
+static long gettid(void) { return syscall(__NR_gettid); }
+#endif
 
 void gpr_log(const char* file, int line, gpr_log_severity severity,
              const char* format, ...) {