[clang][test] Specify value of -fopenmp=libomp for test. (#150301)

`libomp` is the default value when unconfigured in cmake, but llvm can
be configured to have `libgomp` be the default instead. Explicitly
specify this value so the test does not fail when it assumes libomp is
always the default.

Author: rupprecht

clang/test/Driver/offload-target.c

@@ -10,7 +10,7 @@
 // RUN: | FileCheck %s -check-prefix=CUDA
 // CUDA: "nvptx64-nvidia-cuda" - "clang", inputs: ["[[INPUT:.+]]"], output: "[[NV_OBJ:.+]]"
 
-// RUN: %clang -### --offload-targets=amdgcn-amd-amdhsa,nvptx64-nvidia-cuda -fopenmp \
+// RUN: %clang -### --offload-targets=amdgcn-amd-amdhsa,nvptx64-nvidia-cuda -fopenmp=libomp \
 // RUN:   -Xarch_amdgcn --offload-arch=gfx90a -Xarch_nvptx64 --offload-arch=sm_89 \
 // RUN:   -nogpulib -nogpuinc %s -ccc-print-bindings 2>&1 \
 // RUN: | FileCheck %s -check-prefix=OPENMP