[compiler-rt] [runtimes] Allow building against an installed LLVM tree (PR #86209)

[via llvm-commits]

huixie90 wrote:

Basically the steps in
https://libcxx.llvm.org/VendorDocumentation.html#vendordocumentation
https://libcxx.llvm.org/TestingLibcxx.html

```
cmake -G "Unix Makefiles" -S runtimes -B build_flat_multimap -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" -DLIBCXX_INCLUDE_TESTS:BOOL=TRUE -DLIBCXX_TEST_PARAMS="enable_modules=clang" -DCMAKE_CXX_COMPILER="/usr/local/bin/clang++" -DPython3_EXECUTABLE="/usr/local/bin/python"

cd build_flat_multimap
make cxx cxxabi cxx_experimental -j12
make cxx-test-depends
./bin/llvm-lit -sv  --param std=c++23  ../libcxx/test/std
```

The output of `make -n check-all`
```
make -n check-all
make: *** No rule to make target `check-all'.  Stop.
```

https://github.com/llvm/llvm-project/pull/86209