0.8.19.1: PowerPC linkage tables (darwin only for now)

0.8.19.1: PowerPC linkage tables (darwin only for now)
   * Linkage tables are now implemented on PowerPC.
     Right now I've only enabled it on Darwin; to enable it on Linux,
	 the addresses used for linkage table spaces would need to be tested.
   * Lazy foreign functions do not work correctly - the code expects the signal
     that occurs when jumping to a write-protected page to be the same as the
	 signal that occurs when reading from one. On Darwin the former is SIGBUS
	 and the latter is SIGSEGV; this means that the fault address testing does
	 not work on Darwin.