Do not emit every iface load/codegen event to stderr when using -v by default
Not only does it clutter the output horribly, it also seems to overly much impact performance.
ghc 8.6.5 normal: 2.4s ghc 8.6.4 -v : 2.5s ghc head normal : 2.4s ghc head -v : 4s
This seems a bit excessive, especially when we don't actually write an eventlog.
I suspect the main cost is all the IO (44k lines for head!) so just avoid this output by default while still writing to the eventlog is likely fine.