When the idle proc blocks while executing some work, another proc
takes over the idling.  This implementation is optimized for
non-blocking work.

(reproducing mapbench behavior in userspace)

memory allocators.  this fixes a bug that caused gdb to crash..

now utest gets good scalability for random add/remove operations on a
1024-element crange:

nickolai@ben:~/proj/xv6$ for n in 1 2 4 8 16 32 64 80; do ( time ./o.user/utest -n $n ) 2>&1 | grep ^real | awk '{print "'$n'", $2;}'; done
1 0m21.570s
2 0m16.342s
4 0m9.997s
8 0m5.484s
16 0m3.218s
32 0m1.978s
64 0m1.741s
80 0m1.814s
nickolai@ben:~/proj/xv6$

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.