Bring the 32-bit powerpc (PowerMac) back to live. The commit 338486 reworked

Bring the 32-bit powerpc (PowerMac) back to live. The commit 338486 reworked
some TLS bits. This broke operation on the PowerMac. Namely one could not login.
At login the screen/shell was giving back lots of backslashes and the login
shell dumped core.

The fix to this issue is to revert the powerpc commit from 338486 and to
increase the TLS_TCB_SIZE to 16.
Reverting only did not help, login was possible but userland applications
aborted with strange messages.

I tested this patch with world/kernel builds and with port upgrades.
Additionally a full gcc8 bootstrap was successfully completed.

Reviewed by: jhibbits@
Approved by: re (Glen)