Validate the assembly code that Subzero generates in unit tests.

Add RUN lines to applicable lit tests to pipe the output of Subzero (in
-Om1 and/or -O2 mode) to llvm-mc for validation.

Note that the following unit tests fail the validation:
 * callindirect.pnacl.ll
 * mangle.ll
 * nacl-other-intrinsics.ll

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/411693003