Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Merge gdtoa-20110304.

Import gdtoa sources dated 2011-03-04. This version includes a number
of bugfixes, although I believe we already have local patches for the
ones people are likely to notice.

Per a request from arundel@, I also added the vendor's change log,
which is available separately from ftp://ftp.netlib.org/fp/.