[Offload][AMDGPU] LLVM_ENABLE_RUNTIMES=flang-rt for amdgpu-offload-* (#129692)

Enable the LLVM_ENABLE_RUNTIMES=flang-rt build of the Fortran runtime
for the amdgpu-offload-* buildbots. This pre-population cmake cache
files is referred to by the llvm-zorg annotated builder factory
[script](https://github.com/llvm/llvm-zorg/blob/872f477610d83821c9f1368c969006789b21011b/zorg/buildbot/builders/annotated/amdgpu-offload-cmake.py#L26).

The corresponding change in llvm-zorg is
llvm/llvm-zorg#402

Author: Meinersbur

offload/cmake/caches/AMDGPUBot.cmake

@@ -10,7 +10,7 @@ set(CMAKE_C_COMPILER_LAUNCHER ccache CACHE STRING "")
 set(CMAKE_CXX_COMPILER_LAUNCHER ccache CACHE STRING "")
 
 set(LLVM_ENABLE_PROJECTS "clang;lld;mlir;flang" CACHE STRING "")
-set(LLVM_ENABLE_RUNTIMES "compiler-rt;openmp;offload" CACHE STRING "")
+set(LLVM_ENABLE_RUNTIMES "compiler-rt;openmp;offload;flang-rt" CACHE STRING "")
 
 set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ON CACHE BOOL "")
 set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")