Hadrian: refactor .o and .hi rules into a single rule that produces both files

Refactor the rules for .hi and .o into a single rule using &%>

Currently the rule for .hi files just triggers (via need) the rule for the .o file, and .o rule generates both the .o and .hi file. Likewise for .o-boot and .hi-boot files. This is a bit of an abuse of Shake, and in fact shake supports rules with multiple output with the &%> function. This exact use case appears in Neil Mitchell's paper Shake Before Building section 6.3.