Use native methods to implement absl::base_internal::GetPID() on

FreeBSD, NetBSD, and OpenBSD

https://man.freebsd.org/cgi/man.cgi?query=pthread_getthreadid_np
https://man.netbsd.org/_lwp_self.2
https://man.openbsd.org/getthrid.2

This fixes a build break caused by
https://github.com/abseil/abseil-cpp/commit/88cc63ef739d83277b492e881be72e9069fcb1fe

Fixes #1518

PiperOrigin-RevId: 563200172
Change-Id: Ifd1b65c84e3631075248bc2e01b8f047dc72d201

absl/base/internal/sysinfo.cc

@@ -34,6 +34,14 @@
 #include <sys/sysctl.h>
 #endif
 
+#ifdef __FreeBSD__
+#include <pthread_np.h>
+#endif
+
+#ifdef __NetBSD__
+#include <lwp.h>
+#endif
+
 #if defined(__myriad2__)
 #include <rtems.h>
 #endif
@@ -421,7 +429,7 @@ pid_t GetTID() {
   return tid;
 }
 
-#elif defined(__APPLE__)
+#elif defined(__APPLE__) || defined(__FreeBSD__)
 
 pid_t GetTID() {
   uint64_t tid;
@@ -432,6 +440,14 @@ pid_t GetTID() {
   return static_cast<pid_t>(tid);
 }
 
+#elif defined(__OpenBSD__)
+
+pid_t GetTID() { return getthrid(); }
+
+#elif defined(__NetBSD__)
+
+pid_t GetTID() { return static_cast<pid_t>(_lwp_self()); }
+
 #elif defined(__native_client__)
 
 pid_t GetTID() {