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.

src/thread/powerpc64/syscall_cp.s

@@ -34,4 +34,11 @@ __cp_end:
 	blr
 
 __cp_cancel:
+	mflr 0
+	bl 1f
+	.long .TOC.-.
+1:	mflr 3
+	lwa 2, 0(3)
+	add 2, 2, 3
+	mtlr 0
 	b __cancel