lucetc CLI opt-level options are missing

[acfoltzer]

--opt-level {0,1,2} work fine, but none of the other options are recognized despite being described in the help text.

It'd probably also be nice to allow a short option from, e.g., -O2 if we're going to keep up the analogy with gcc/clang.

[awortman-fastly]

https://github.com/fastly/lucet/pull/328 adds long form names for opt-level names, but no -O2 and friends yet.