Ruby

Add a realpath cache to reduce number of syscalls.

Number of lstat and stat syscalls for each 'require'd file is doubled,
because rb_realpath_internal is called from two places with the same
arguments in require_internal; once for checking the realpaths cache,
and once in load_iseq_eval when iseq is not found.

Introduce rb_realpath_internal_cached function to reuse the realpath_map
cache which memoizes rb_realpath_internal function, leading to less
syscalls and increased startup performance depending on the cost of the syscalls
in a particular environment.