this is not a public interface, and does not even necessarily match
the syscall on all archs that have a syscall by that name.

on archs where it's implemented in C, no action on the source file is
needed; the hidden declaration in pthread_arch.h suffices.

these are not a public interface and are not intended to be callable
from anywhere but the public clone function or other places in libc.

entering the local entry point for __cancel from __cp_cancel is valid
if __cp_cancel was reached from __syscall_cp, since both are in libc
and share the same TOC pointer, but it is not valid if __cp_cancel was
reached when cancel_handler rewrote the program counter for
asynchronous cancellation of code outside libc.

to ensure __cancel is entered with a valid TOC pointer, recompute the
correct value in a PC-relative manner before jumping.