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, since it requires some
changes to the context switching code I can't test. What is needed
should be approximately:
(1) Call PAL_wdunique in cpu_switchto to ensure that the value is
updated.
(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