[jcw-gen] Build jcw-gen.exe into $(UtilityOutputFullPath)

[jonpryor]

Context: https://github.com/mono/Embeddinator-4000
Context: b6431ac

Embeddinator-4000 is an in-progress utility to easily allow
Java/C/Objective-C/etc. to consume .NET CIL. For example, it would be
desirable to be able to "export" a .dll into an Android .aar file,
so that Java developers using Android Studio can consume and use
managed assemblies in a straightforward manner.

Part of making this work will be to generate Java "bindings" of an
assembly.

"Funny" thing: we already partially have that: jcw-gen.exe will take
an assembly and generate Java Callable Wrappers for Java.Lang.Object
subclasses from that assembly. Aside from the base class requirement,
this is (more or less) what Embeddinator-4000 requires...except that
Embeddinator-4000 wants to handle all managed types, not just
Java.Lang.Object subclasses.

That said, at some point it will be desirable for "transparent" use of
Java.Lang.Object subclasses; Embeddinator-4000 can't (reasonably or
sanely) use it's "normal" non-Java.Lang.Object-aware generator to
bind Java.Lang.Object subclasses; for starters, the base class will
be completely wrong (Java.Lang.Object instead of
java.lang.Object!).

Embeddinator-4000 could use the
Java.Interop.Tools.JavaCallableWrappers assembly in order to perform
this work, but that would require that Embeddinator-4000 reference
Java.Interop or otherwise "obtain" an assembly to build against,
complicating bootstrapping and ongoing maintenance.

Instead, Embeddinator-4000 can use our existing command-line code
generation interface, jcw-gen.exe. This simplifies the build --
at the expense of runtime, as now you need to specify where
jcw-gen.exe is -- and provides a "break" between repositories.

Update jcw-gen.csproj so that it's $(OutputPath) is
$(UtilityOutputFullPath), as is the case for class-parse,
generator, and logcat-parse, so that jcw-gen.exe can be
distributed as part of Xamarin.Android.