Add GetTID() implementation for Fuchsia

The GetTID() now uses the value of the current thread handle from
zx_thread_self() on Fuchsia for the thread ID. This is better than the default
implementation which uses pthread_self(), since that results in a 64-bit
pointer being truncated into the 32-bit pid_t type.

PiperOrigin-RevId: 660270084
Change-Id: I7c8b4a26cca775be62eb6f6926cb734cbb893585

absl/base/internal/sysinfo.cc

@@ -46,6 +46,10 @@
 #include <rtems.h>
 #endif
 
+#if defined(__Fuchsia__)
+#include <zircon/process.h>
+#endif
+
 #include <string.h>
 
 #include <cassert>
@@ -461,6 +465,16 @@ pid_t GetTID() {
   return reinterpret_cast<pid_t>(thread);
 }
 
+#elif defined(__Fuchsia__)
+
+pid_t GetTID() {
+  // Use our thread handle as the TID, which should be unique within this
+  // process (but may not be globally unique). The handle value was chosen over
+  // a kernel object ID (KOID) because zx_handle_t (32-bits) can be cast to a
+  // pid_t type without loss of precision, but a zx_koid_t (64-bits) cannot.
+  return static_cast<pid_t>(zx_thread_self());
+}
+
 #else
 
 // Fallback implementation of `GetTID` using `pthread_self`.