Reduce the number of directories listed in libdirs, which makes the linker
check a lot less dead ends looking for directories.
Eliminated some directories that didn't really contain shared libraries,
or only contained the linker.
That left only 2, one in lib and one in usr/lib, so consolidate those two.
Doing it this way, rather than just consolidating all libs that might exist
into a single directory means that, if there are optimised versions of some
libs, eg in lib/subarch/foo.so, and lib/subarch2/foo.so, they don't get
moved around in a way that would make the linker pick the wrong one.
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