[runtimes] Calculate the path to the llvm directory without the LLVM cmake package

If building by pointing cmake directly at the llvm-project/runtimes
directory, the llvm cmake package files (that provide e.g.
LLVM_BUILD_MAIN_SRC_DIR) aren't necessarily available. Instead just
use a path relative to the current source dir.

Differential Revision: https://reviews.llvm.org/D109717