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 MIPS part is not complete at the moment, since it requires some
changes I can't test. What is needed should be approximately:
(1) Add cpu_lwp_setprivate to update HWR 29 for the current thread, if
supported by the CPU (e.g. it is not emulated in the trap code).
(2) Set the register in cpu_switchto.
(3) Consider adding a fast path for this instruction in the trap code.
(4) 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