1.0.16.30: couple of small Darwin cleanups

1.0.16.30: couple of small Darwin cleanups

 * Add -lpthread os OS_LIBS on x86 builds as well, since malloc()
   may not be thread-safe without it (but with it it apparently should
   be...)

 * Pass -arch x86_64 to the C compiler in foreign.test.sh when doing
   64 bit builds on Darwin, since x86 is the default there.