- ELF header bits and addresses
  - do not let instructions span 32-byte boundary
  - align CALLs so return is to 32-byte boundary
  - align indirect jump targets to 32-byte boundary
    (only possible indirect jumps are function entries)

still to do:
  - replace indirect jump, indirect call, and ret with
    nacl-approved instruction sequences
  - switch to GS segment for m-local storage

R=ken
OCL=34818
CL=34818