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

src/IceFixups.cpp

diff --git a/src/IceFixups.cpp b/src/IceFixups.cpp
index 05559c5..b6adf84 100644
--- a/src/IceFixups.cpp
+++ b/src/IceFixups.cpp
@@ -46,13 +46,15 @@
   return Str.str();
 }
 
-void AssemblerFixup::emit(GlobalContext *Ctx) const {
+void AssemblerFixup::emit(GlobalContext *Ctx, RelocOffsetT BaseOffset) const {
+  if (!ALLOW_DUMP)
+    return;
   Ostream &Str = Ctx->getStrEmit();
-  // TODO(jvoung): Not yet clear how to emit a relocation against
-  // the null symbol.
-  assert(!isNullSymbol());
-  Str << symbol(Ctx);
-  RelocOffsetT Offset = offset();
+  if (isNullSymbol())
+    Str << "__Sz_AbsoluteZero";
+  else
+    Str << symbol(Ctx);
+  RelocOffsetT Offset = offset() + BaseOffset;
   if (Offset)
     Str << " + " << Offset;
 }