Subzero: Support non-IRT immediate calls with -filetype=iasm.

This is done by emitting the following:
  .byte 0xe8
  .long __Sz_AbsoluteZero -4 - .

The linker is passed an argument like
  --defsym=__Sz_AbsoluteZero=0
to force the symbol's value to 0.

This special symbol is needed due to an llvm-mc parsing bug.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1027593002