[CMake] Use `LLVM_COMMON_CMAKE_UTILS` in runtimes just for clarity

In D116472 we created conditionally defined variables for the tools to
unbreak the legacy build where they are in `llvm/tools`.

The runtimes are not tools, so that flexibility doesn't matter. Still,
it might be nice to define (unconditionally) and use the variable for
the runtimes simply to make the code a bit clearer and document what is
going on.

Also, consistently put project dirs at the beginning, not end of `CMAKE_MODULE_PATH`. This ensures they will properly shadow similarly named stuff that happens to be later on the path.

Reviewed By: mstorsjo, #libunwind, #libc, #libc_abi, ldionne

Differential Revision: https://reviews.llvm.org/D116477