Subzero: Separate "\t" from "opcode" in asm emission.

Instead of output like this:
  Str << "\tmov\t" << ...
we prefer to use string concatenation:
  Str << "\t" "mov\t" << ...

That way, "git grep -w mov" can be used for more precise pattern matching.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1523873003 .