get rid of __sync_synchronize in gc_begin_epoch, since a LOCK instruction
acts as an mfence.

useful ref: http://www.cl.cam.ac.uk/~pes20/weakmemory/cacm.pdf

Otherwise we leave junk dirents on unlink.

which allows moving initsperf() to before initkalloc().

..this turned out to be more complicated than I expected.