Add C-only implementation of runfiles library

The `//cc/runfiles:runfiles` library is implemented in C++, which means depending on it requires linking against `libstdc++`. This can be unwanted when linking a program that doesn't otherwise have a C++ dependency.

It would be nice to have a `runfiles.c` somewhere that implements the same logic, but on top of the C stdlib.

Related: https://github.com/jmillikin/rules_bison/issues/19