patch 9.0.2028: confusing build dependencies

patch 9.0.2028: confusing build dependencies

Problem:  confusing build dependencies
Solution: clean them up, make them parallelizable

Separate vim binary and unittest dependencies, make them parallelizable

Clean up make dependencies so Vim and unit test binaries only depend on
the object files they need. This fixes an existing issue where after
running unit tests, the Vim binary would be invalidated, which results
in it having to be linked again when running script tests, even though
Vim was already previously built.

Make link.sh (script we use to link those binaries) generate namespaced
temporary files for each app to avoid them colliding with each other.
This allows `unittesttargets` to be built in parallel.

These fixes are useful when using link-time-optimization as the link
phase could now take minutes rather than a few seconds.

closes: #13344

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>