%r26 hasn't been used by the threads implementation since 2016, so stop
resetting it in child_return() and update the comment in tcb.h to reflect
reality

ok miod@ aoyama@

Make this build (and run) with gcc 4.

TCB_GET_MEMBER() is no longer used after the TIB changes

It's been a quarter century: we can assume volatile is present with that name.

ok dlg@ mpi@ deraadt@

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
purpos

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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Specify the TLS variant for each platform.