[Libomptarget] Fix multiply defined symbol during linking

[jhuber6]

Summary

This patch adds the weak identifier to the openmp device environment
variable. The changes introduced in https://reviews.llvm.org/D117211
result in multiply defined symbols. Because the symbol is potentially
included multiple times for each offloading file we will get symbol
colisions, and because it needs to have external visiblity it should be
weak.

[JonChesterfield]

Sure, it can be weak. Definition in the old devicertl too please. I guess we have multiple copies because there are multiple TUs each with their own internalized RTL, and this symbol needs to exist in isolation so the host runtime only has to patch one of them. Thanks!

[jhuber6]

Adding changes to old runtime.

[JonChesterfield]

Thanks!

openmp/libomptarget/DeviceRTL/src/Configuration.cpp

 using namespace _OMP;
 
 #pragma omp declare target
 
 extern uint32_t __omp_rtl_debug_kind; // defined by CGOpenMPRuntimeGPU
 
 // TODO: We want to change the name as soon as the old runtime is gone.
 DeviceEnvironmentTy CONSTANT(omptarget_device_environment)
-    __attribute__((used, retain));
+    __attribute__((used, retain, weak));
 
 uint32_t config::getDebugKind() {
   return __omp_rtl_debug_kind & omptarget_device_environment.DebugKind;
 }
 
 uint32_t config::getNumDevices() {
   return omptarget_device_environment.NumDevices;
 }

openmp/libomptarget/deviceRTLs/amdgcn/src/target_impl.h

 
 typedef uint64_t __kmpc_impl_lanemask_t;
 
 #define INLINE inline
 #define NOINLINE __attribute__((noinline))
 #define ALIGN(N) __attribute__((aligned(N)))
 #define PLUGIN_ACCESSIBLE                                                      \
   __attribute__((used))   /* Don't discard values the plugin reads */          \
+  __attribute__((weak))   /* We may have multiple definitions */               \
   __attribute__((retain)) /* Also needed to keep values alive */               \
   __attribute__((visibility("default"))) /* Access via SHT_HASH */             \
   __attribute__((section(".data")))      /* Not .bss, can write before load */
 
 #include "llvm/Frontend/OpenMP/OMPGridValues.h"
 
 INLINE constexpr const llvm::omp::GV &getGridValue() {
   return llvm::omp::getAMDGPUGridValues<__AMDGCN_WAVEFRONT_SIZE>();