gh-111962: Make dtoa thread-safe in `--disable-gil` builds. (#112049)

This updates `dtoa.c` to avoid using the Bigint free-list in --disable-gil builds and
to pre-computes the needed powers of 5 during interpreter initialization.

* gh-111962: Make dtoa thread-safe in `--disable-gil` builds.

This avoids using the Bigint free-list in `--disable-gil` builds
and pre-computes the needed powers of 5 during interpreter initialization.

* Fix size of cached powers of 5 array.

We need the powers of 5 up to 5**512 because we only jump straight to
underflow when the exponent is less than -512 (or larger than 308).

* Rename Py_NOGIL to Py_GIL_DISABLED

* Changes from review

* Fix assertion placement