[libc][rpc] Land helpers from D148943

Author: JonChesterfield

libc/src/__support/RPC/rpc_util.h

@@ -26,6 +26,16 @@ LIBC_INLINE void sleep_briefly() {
 #endif
 }
 
+/// Get the first active thread inside the lane.
+LIBC_INLINE uint64_t get_first_lane_id(uint64_t lane_mask) {
+  return __builtin_ffsl(lane_mask) - 1;
+}
+
+/// Conditional that is only true for a single thread in a lane.
+LIBC_INLINE bool is_first_lane(uint64_t lane_mask) {
+  return gpu::get_lane_id() == get_first_lane_id(lane_mask);
+}
+
 } // namespace rpc
 } // namespace __llvm_libc