Modify signals available to getsignals on WIN32

[jstuder-gh]

JS: All the signals we need for Windows are actually already defined in os.constants.signals

JVM: Match getsignals JVM with MoarVM/JS backends

[usev6]

@jstuder-gh sorry I've missed your request for a review back in 2018. I don't use Windows, but as far as I can see, adding the signals to sigWanted on the JVM backend increases consistency and looks sensible. nqp::getsignals will return negative values for all the not supported signals anyway, right? I can't comment on the changes to the JS backend.

The merge conflict stems from renaming s/perl6/raku/ earlier today (https://github.com/Raku/nqp/pull/630). It shouldn't be hard to resolve -- the changes need probably just copied over to the new location src/vm/jvm/runtime/org/raku/nqp/runtime/IOOps.java.