Fix GetTID() on FreeBSD

https://github.com/abseil/abseil-cpp/issues/1518#issuecomment-1709098904
pointed out that the previous untested fix doesn't work because
pthread_getthreadid_np() has a different signature on Darwin.

Follow up to https://github.com/abseil/abseil-cpp/commit/b9707b7d7845f9710ae6d5906827b833fdcc2754

Fixes #1518

PiperOrigin-RevId: 563432451
Change-Id: Id0a9212e9c4413fa520a42934efaed2a06ca5dbc

absl/base/internal/sysinfo.cc

@@ -429,7 +429,7 @@ pid_t GetTID() {
   return tid;
 }
 
-#elif defined(__APPLE__) || defined(__FreeBSD__)
+#elif defined(__APPLE__)
 
 pid_t GetTID() {
   uint64_t tid;
@@ -440,6 +440,10 @@ pid_t GetTID() {
   return static_cast<pid_t>(tid);
 }
 
+#elif defined(__FreeBSD__)
+
+pid_t GetTID() { return static_cast<pid_t>(pthread_getthreadid_np()); }
+
 #elif defined(__OpenBSD__)
 
 pid_t GetTID() { return getthrid(); }