BUILD: makefile: do not modify the build options during make reg-tests

I'm quite fed up with having to rebuild everything from scratch after each
and every "make reg-tests", especially during bisects. The only reason for
this is that there are no build options passed to make for reg-tests, which
modifies the .build_opts file again, resulting in a change upon next build.
Let's just keep this file out of the dependency check for make reg-tests.
1 file changed