Add scalar lowering for sqrt intrinsic.

Re-used test_arith_main.cpp, mostly to share the set of interesting
floating point constants.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3882
R=stichnot@chromium.org, wala@chromium.org

Review URL: https://codereview.chromium.org/384443003
10 files changed