This enables the following workflow:

1. Run "szbuild.py --filetype=asm ..." to generate a .s file.
2. Edit the .s file to mock up a Subzero change.
3. Run "szbuild.py --filetype=asm --no-sz ..." to restart the build post pnacl-sz.

This workflow is good for trying out localized changes to hot basic blocks, though it isn't really appropriate for global optimizations like register allocation.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1417003005 .