[Libomptarget] Fix multiply defined symbol during linking

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.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D117231

Author: jhuber6

openmp/libomptarget/DeviceRTL/src/Configuration.cpp

@@ -24,7 +24,7 @@ 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;

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

@@ -32,6 +32,7 @@ typedef uint64_t __kmpc_impl_lanemask_t;
 #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 */