Implement LSR instructions for the integrated ARM assembler.

Also factors out the body of method lsl() to emitShift(), so
that all forms of shift instructions can use the same code.

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

Review URL: https://codereview.chromium.org/1501073002 .
6 files changed