gdtoa: fix endianness issue on powerpc64le

The current code assumes running on big-endian, which causes issues with
e.g. strtod() as reported on https://github.com/ocaml/ocaml/pull/10837#issuecomment-1605346422
This is probably a leftover from when powerpc64le was introduced.

Approved by: jhibbits (on IRC)