[Libomptarget] Work around bug in initialization of libomptarget

Libomptarget uses some shared variables to track certain internal stated
in the runtime. This causes problems when we have code that contains no
OpenMP kernels. These variables are normally initialized upon kernel
entry, but if there are no kernels we will see no initialization.
Currently we load the runtime into each source file when not running in
LTO mode, so these variables will be erroneously considered undefined or
dead and removed, causing miscompiles. This patch temporarily works
around the most obvious case, but others still exhibit this problem. We
will need to fix this more soundly later.

Fixes #54208.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D121007

openmp/libomptarget/DeviceRTL/src/Mapping.cpp

@@ -258,7 +258,10 @@ uint32_t mapping::getNumberOfProcessorElements() {
 /// Execution mode
 ///
 ///{
-static int SHARED(IsSPMDMode);
+
+// TODO: This is a workaround for initialization coming from kernels outside of
+//       the TU. We will need to solve this more correctly in the future.
+int __attribute__((used, retain, weak)) SHARED(IsSPMDMode);
 
 void mapping::init(bool IsSPMD) {
   if (mapping::isInitialThreadInLevel0(IsSPMD))