Implement GetTID for NACL platform.

in NACL pthread_self() returns a pointer that isn't directly convertible to arithmetic type.

PiperOrigin-RevId: 539023020
Change-Id: I3745ec5565f3a99ccb1d9df12c27a80e57ca4755

absl/base/internal/sysinfo.cc

@@ -426,6 +426,15 @@ pid_t GetTID() {
   return static_cast<pid_t>(tid);
 }
 
+#elif defined(__native_client__)
+
+pid_t GetTID() {
+  auto* thread = pthread_self();
+  static_assert(sizeof(pid_t) == sizeof(thread),
+                "In NaCL int expected to be the same size as a pointer");
+  return reinterpret_cast<pid_t>(thread);
+}
+
 #else
 
 // Fallback implementation of `GetTID` using `pthread_self`.