Adding support for TLS/NPTL for Xtensa

[Chris Zankel]

Hi,

Baruch Siach and I have been working on supporting TLS and NPTL for the 
Xtensa architecture, and I would like to start committing those changes.

Most changes are very limited to the Xtensa architecture, so shouldn't 
affect other architectures at all. However, the Xtensa implementation 
requires TLS descriptor support, which Baruch has ported to uClibc, and 
it touches shared code. We'll post that patch for review.

I understand that there have been requests for a next release, and it 
would be great if we could get NPTL support for Xtensa into that 
release. Some library functions are only implemented in NPTL and not the 
old linuxthreads library, so I see failures when building some packages 
with buildroot.

Please let me know if you have any objects or suggestions.

Thanks,
-Chris