Add function to get libjulia path at runtime

This adds a function "jl_get_libjulia_path" that gives the path of the libjulia library at runtime.

I would propose that this path should be the first location where the sys.ji (or sys.so in the future) is searched (in jl_init and repl.c). Once the static compilation branch is merged I can make a PR for changes in jl_init and repl.c

In this PR only the addition of the function is included.