Remove obsolete Tru64 UNIX V5.1B support

[Jonathan Wakely]

On 5 March 2012 17:01, Rainer Orth wrote:
>
> * The libstdc++ testsuite is messy since every thing pthread test
>  includes the complete list of targets where it should be run, and the
>  options required.  I've long meant to clean this up, but this will
>  have to wait until after osf and irix are gone from the tree.

That would be fantastic if you do clean it up some time.

The libstdc++ parts of this patch are OK.