Redesign interface to machine-dependent code to cleanse lwp.c of

Redesign interface to machine-dependent code to cleanse lwp.c of
machine-dependencies:
(lwpNewContext): New, factored out of lwpCreate().
(lwpSwitchContext): New, factored out of lwpReschedule().
(LWP_EXTRASTACK, STKALIGN, lwpInitContext, lwpSave, lwpRestore): Move
to arch.c.  Remove for UCONTEXT, else static linkage unless macro or
AIX32.

(lwpReschedule): Skip lwpStatus() when not actually switching.

(LwpStackGrowsDown): New.
(lwpInitSystem): Initialize it.
(lwpStackCheck, lwpStackCheckUsed): Use it.