* change ldt0setup to set GS itself; nacl won't let us do it.
  * change breakpoint to INT $3 so 8l can translate to HLT for nacl.
  * panic if closure is needed on nacl.
  * do not try to access symbol table on nacl.
  * mmap in 64kB chunks.

nacl support:
  * system calls, threading, locks.

R=r
DELTA=365  (357 added, 5 deleted, 3 changed)
OCL=34880
CL=34906