[OpenMP] Fix runtimes default build (#149871) The default build of openmp (`cmake -S <llvm-project>/runtimes -DLLVM_ENABLE_RUNTIMES=openmp`) current fails with ``` CMake Error at /home/meinersbur/src/llvm/flangrt/_src/cmake/Modules/GetClangResourceDir.cmake:17 (string): string sub-command REGEX, mode MATCH needs at least 5 arguments total to command. Call Stack (most recent call first): /home/meinersbur/src/llvm/flangrt/_src/openmp/CMakeLists.txt:126 (get_clang_resource_dir) ``` The reason is that because it is not a bootstrapping-build, the clang resource dir that it intends to write files such as `omp-tools.h` into, is unavailable. Using the Clang resource dir for writing files is conceptually broken, as that dir might be located in `/usr/lib/clang/<version>/`. Writing to it is only intended in bootstrapping builds where Clang is built alongside openmp. This patch unifies the identification of being in a bootstrapping built. The same `LLVM_TREE_AVAILABLE` definition is going to be used in #137828. No reason for each runtime to define its own.
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