Implement clock_gettime(CLOCK_MONOTONIC) properly

It's bad to pretend to support every clock - we should correctly return -1 with EINVAL for the clocks we don't implement. It's also nice to give implementations a way to detect if they have access to a monotonic clock. For comparison, old Linux machines (RHEL 3 for example) don't have CLOCK_MONOTONIC, new ones do.
this name sounds too close to "get_now, monotonically". How about `emscripten_is_get_now_monotonic`? (at the risk of a longer name...) cc @juj for thoughts