Add orr (register) and orr (immediate) to ARM integrated assembler.

Also cleans up comments about handling a rotated imm8 value.

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

Review URL: https://codereview.chromium.org/1412923006 .
6 files changed