Add a flag to use the integrated assembler.

Handle simple XMM binary op instructions as an initial example of how
instructions might be handled. This tests fixups in a very limited sense --
Track buffer locations of fixups for floating point immediates.

Patchset one shows the original dart assembler code (revision 39313), so that
it can be diffed.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/574133002