Adds a notion of a hybrid assembler. That is, if the integrated
assembler can lower an instruction to bytes, it does. Otherwise, it
uses the standalone assembler to generate text as the placeholder for
the instruction. This is done using a textual fixup in the assembly
buffer.

The advantage of the hybrid assembler is that one can incrementally
implement the integrated assembler and still test the generated
assembly.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4334
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1418523002 .