[OpenMP] Fix runtimes default build

[Meinersbur]

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.