Removing the glibc NaCl port

[Florian Weimer]

I just realized that my pipe2 removal probably breaks the NaCl port.

Can we remove it?  It blocks useful cleanups which are compatible with 
the out-of-tree Hurd port.

I don't think anyone has ever reported test results for release builds. 
scripts/build-many-glibcs.py lacks support for it, so we don't even know 
if it still builds.

I hope this proposal is not too controversial.

Thanks,
Florian