Create a new header lwp_private.h to contain _lwp_getprivate_fast,
_lwp_gettcb_fast, _lwp_settcb and remove them from mcontext.h, so that:
1. we don't need special hacks to hide them
2. we can include <lwp.h> where needed to get the necessary prototypes
without redefining them locally.

branches: 1.1.2;
Add a port to the Qemu m68k "virt" platform, a 68040-based virtual machine
with VirtIO I/O devices.

This is a work-in-progress, but "successfully mounts root file system
and then complains about missing /sbin/init that is, in fact, not there"
seems like a good checkpoint.