Make -Z gen-crate-map the default when compiling staticlib

[alexcrichton]

Not having a toplevel crate map when compiling a staticlib is almost useless because anything that uses the crate map will not work. Might as well just generate a toplevel crate map like we do for executables because a staticlib is an end produce in the same sense.

[alexcrichton]

Hm, now I remember why I didn't do this before. It's tricky in that when you have multiple output formats not all of them will want the toplevel crate map. Perhaps this could use the same trick as metadata in that there's a separate object file for it, but I think that's overkill. Closing.