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
diff --git a/src/IceFixups.h b/src/IceFixups.h
index 1bc45f3..3b2b524 100644
--- a/src/IceFixups.h
+++ b/src/IceFixups.h
@@ -44,7 +44,7 @@
 
   void set_value(const Constant *Value) { value_ = Value; }
 
-  void emit(GlobalContext *Ctx) const;
+  void emit(GlobalContext *Ctx, RelocOffsetT BaseOffset) const;
 
 private:
   intptr_t position_;