Clean up whitespace in Makefiles.

While working on issue 3326, I noticed a couple of useful conventions
that were not being applied uniformly:

 * Using tabs to put the line-continuation mark (\) in the same column

 * Using two tabs (instead of one) when listing target dependencies
   on multiple lines (to visually offset them from the target commands,
   which are prefixed by one tab).

 * Separating the target commands from the target dependencies by a
   (tab-free) newline (again, so that the two are visually offset).

These conventions seem to be remarkably effective, at least for me,
at make the makefiles more readable.

So even though I know we are getting rid of make and going to be using
rustpkg at some point in the (near?) future, I figured I might as well
do some drive-by cleanup, if for no other reason than it might make
the rustpkg port easier (maybe)?