Use getthrid(2) as GetTID() implementation.

Index: absl/base/internal/sysinfo.cc
--- absl/base/internal/sysinfo.cc.orig
+++ absl/base/internal/sysinfo.cc
@@ -441,6 +441,12 @@ pid_t GetTID() {
   return reinterpret_cast<pid_t>(thread);
 }
 
+#elif defined(__OpenBSD__)
+
+pid_t GetTID() {
+  return getthrid();
+}
+
 #else
 
 // Fallback implementation of `GetTID` using `pthread_self`.