Refactor the rules for .hi and .o into a single rule using &%> Trac #16764
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.