Get TCB address using the RDHWR instruction instead of __get_tcb().
This gives fast access to the address on systems that implement
the UserLocal register. TCB caching is still used when running
in the single-threaded mode in order not to penalize old systems.

The kernel counterpart of this change must be in place before
using this diff!

With guenther@

Make TCB address available to userspace via the UserLocal register.
This lets programs get the address without a system call on OCTEON II
and later.

Add UserLocal load emulation for systems that do not implement
the RDHWR instruction or the UserLocal register.

OK guenther@

Provide mips64 with kernel-facing TCB_{GET,SET} macros that store it
in struct mdproc. With that, all archs have those and the __HAVE_MD_TCB
macro can be unifdef'ed as always defined.

ok kettenis@ visa@ jsing@

Specify the TLS variant for each platform.