Makefile: Rip out splitandfilterandmergemultiarch, etc.

Now that splitandfilterandmergemultiarch and its callers are no longer
used (solely to build fdopendir), they can be removed.  This also
removed the only references to LDX (formerly LD), LIPO, and PLATFORM,
so their default definitions are removed as well.

TESTED:
Build still works (and tests pass) on all platforms.