fix async thread cancellation on powerpc64

fix async thread cancellation on powerpc64

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.