Fixes a couple of bugs that stopped the ARM integrated assembler from
generating assembly code for any spec2k examples.

Fixes are:

1) Handle conditional branches with no else branch.

2) Fix usage of fixups so that the emit method does any needed buffer
lookups. This fixes case where textual fixups (with zero length)
appear at the end of the assembly file.

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

Review URL: https://codereview.chromium.org/1417173003 .