Ruby 3.2 - Speed up rebuilding the loaded feature index and realpath cache (#8023)

* Speed up rebuilding the loaded feature index

Rebuilding the loaded feature index slowed down with the bug fix
for #17885 in 79a4484a072e9769b603e7b4fbdb15b1d7eccb15.  The
slowdown was extreme if realpath emulation was used, but even when
not emulated, it could be about 10x slower.

This adds loaded_features_realpath_map to rb_vm_struct. This is a
hidden hash mapping loaded feature paths to realpaths. When
rebuilding the loaded feature index, look at this hash to get
cached realpath values, and skip calling rb_check_realpath if a
cached value is found.

Fixes [Bug #19246]

* 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.

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Co-authored-by: Jeremy Evans <code@jeremyevans.net>

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