ppc64: use bl instruction for branching to exit

when I originally wrote this, we used `b exit@GOT`, but this is unnecessary because
we can simply do `bl exit` instead.

fixes #71

Author: kaniini

arch/ppc64/startcontext.S

@@ -28,6 +28,6 @@ FUNC(libucontext_trampoline)
 	li	3,-1
 
 no_linked_context:
-	b	exit@GOT
+	bl	exit
 	nop
 END(libucontext_trampoline)