Add field to tls_tcb to support segmented stacks in LLVM

When segmented stack support is enabled, LLVM adds code in front of
every function to check if the stack is already exhausted, in which
case it calls __morestack. For this reason LLVM needs to know the lower
boundary of the stack to check against the stack pointer.
The stack boundary can be stored in this per-thread field (tcb_segstack)
and accessed via %fs:32 (x86_64) or %fs:16 (i386) from the code generated
by LLVM.
Document that tcb_segstack should not be reordered.