Makefile: add {program,xdiff,test,git,fuzz}-objs & objects targets

Makefile: add {program,xdiff,test,git,fuzz}-objs & objects targets

Add targets to compile the various *.o files we declared in commonly
used *_OBJS variables. This is useful for debugging purposes, to
e.g. get to the point where we can compile a git.o. See [1] for a
use-case for this target.

https://lore.kernel.org/git/YBCGtd9if0qtuQxx@coredump.intra.peff.net/

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>