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.
799f28d7a5