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

[via llvm-commits]

huixie90 wrote:

> > Hello, After this commit, I can no longer run libc++ tests with llvm-lit anymore.
> > > fatal: You seem to be running Lit directly -- you should be running Lit through /bin/llvm-lit, which will ensure that the right Lit configuration file is used. See https://libcxx.llvm.org/TestingLibcxx.html#usage for how to run the libc++ tests.
> 
> Can you send me the commands you are running? Also let me know what `ninja check-all -v -n` prints.

I usually run this under my build directory
```
./bin/llvm-lit -sv  --param std=c++23  ../libcxx/test/std
```
It no longer works.

Also I tried to use the `libcxx/utils/libcxx-lit` it failed with the same error.

But if I revert your commit, everything works again

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