[flang][OpenMP] Add missing implementation for 'omp_is_initial_device' (#83056)

Resolve issue #82047

GitOrigin-RevId: 99335a646bc0b10066d77cec08ae8cab0162efde
diff --git a/libomptarget/DeviceRTL/src/State.cpp b/libomptarget/DeviceRTL/src/State.cpp
index 40f99e0..a1e4fa2 100644
--- a/libomptarget/DeviceRTL/src/State.cpp
+++ b/libomptarget/DeviceRTL/src/State.cpp
@@ -426,6 +426,8 @@
 int omp_get_team_num() { return mapping::getBlockIdInKernel(); }
 
 int omp_get_initial_device(void) { return -1; }
+
+int omp_is_initial_device(void) { return 0; }
 }
 
 extern "C" {