Send universe build log files to .OBJDIR.

[will]

Summary

This honors 'make obj' principles, in which modifications are only made to
the object directory.

Test Plan

Run 'make universe', observe that nothing breaks.

[emaste]

What does ${.OBJDIR} expand to for individual archs in universe?

[will]

In D873#216330, @emaste wrote:

What does ${.OBJDIR} expand to for individual archs in universe?

Does it matter? This isn't affected by the value of .OBJDIR in the per-arch sub-makes. I haven't run this patch in a while, but it seems straightforward to me. It puts the log files in the parent .OBJDIR; all universes are nested just below there. Maybe it makes more sense to put the log file in the individual arch's .OBJDIR instead, but this way keeps the existing aesthetic of all the universe log files being in one place.

[emaste]

In D873#220633, @will wrote:

Does it matter?

Not too much, it's really just a "where do I look for the log files after this change?"