LWP private support / TLS support

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
I've just committed the first major step to TLS support on NetBSD.
The Alpha part is not complete at the moment. I'm not sure if changes to
the context switching code are needed. There is also the issue that the
TLS ABI would currently wire down the SVR4 ABI (usage of %r2). What is
needed to finish this is approximately:
(1) Ensure that %r2 is correctly restored / saved on interrupts and
context switches.
(2) Remove the #if 0 around __HAVE___LWP_GETPRIVATE_FAST in types.h.

After a rebuild of libpthread, the ATF test cases for libpthread should
still work.

Joerg