[Runtimes] Default build must use its own output dirs (#168266)

Post-commit fix of #164794 reported at
https://github.com/llvm/llvm-project/pull/164794#issuecomment-3536253493

`LLVM_LIBRARY_OUTPUT_INTDIR` and `LLVM_RUNTIME_OUTPUT_INTDIR` is used by
`AddLLVM.cmake` as output directories. Unless we are in a
bootstrapping-build, It must not point to directories found by
`find_package(LLVM)` which may be read-only directories. MLIR for
instance sets thesese variables to its own build output
directory, so should the runtimes.

GitOrigin-RevId: c32c1d0d21cedb8017914eb6951bea4cf1fb10f9