Use %r27 as the thread control block pointer, allowing for __get_tcb() calls

in libpthread to be optimized away.

While there, follow DG/UX's example of using more than one register for thread
purposes (after all, the ABI reserves four of them), and also use %r26 to store
a pointer to the current thread's errno.

Since it is not possible to initialize %r26 at thread creation without
intrusive and potentially race-prone changes, have __tfork() reset %r26 to
zero, and libpthread's __cerror lazy initialize it. As soon as %r26 is nonzero,
it will be used instead of calling __errno().

This means that binaries linked against HEAD libpthread need to run on HEAD
kernels, and we are belatedly jumping on the 64-bit time_t bump (since there
are no 64-bit time_t m88k snapshots yet).

Joint work with guenther@; "Your love of asm is sick and wrong, and yet
beautiful." and ok guenther@

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