selftests/bpf: Use auto-dependencies for test objects

Make use of -M compiler options when building .test.o objects to
generate .d files and avoid re-building all tests every time.

Previously, if a single test bpf program under selftests/bpf/progs/*.c
has changed, make would rebuild all the *.bpf.o, *.skel.h and *.test.o
objects, which is a lot of unnecessary work.

A typical dependency chain is:
progs/x.c -> x.bpf.o -> x.skel.h -> x.test.o -> trunner_binary

However for many tests it's not a 1:1 mapping by name, and so far
%.test.o have been simply dependent on all %.skel.h files, and
%.skel.h files on all %.bpf.o objects.

Avoid full rebuilds by instructing the compiler (via -MMD) to
produce *.d files with real dependencies, and appropriately including
them. Exploit make feature that rebuilds included makefiles if they
were changed by setting %.test.d as prerequisite for %.test.o files.

A couple of examples of compilation time speedup (after the first
clean build):

$ touch progs/verifier_and.c && time make -j8
Before: real 0m16.651s
After: real 0m2.245s
$ touch progs/read_vsyscall.c && time make -j8
Before: real 0m15.743s
After: real 0m1.575s

A drawback of this change is that now there is an overhead due to make
processing lots of .d files, which potentially may slow down unrelated
targets. However a time to make all from scratch hasn't changed
significantly:

$ make clean && time make -j8
Before: real 1m31.148s
After: real 1m30.309s

Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/VJihUTnvtwEgv_mOnpfy7EgD9D2MPNoHO-MlANeLIzLJPGhDeyOuGKIYyKgk0O6KPjfM-MuhtvPwZcngN8WFqbTnTRyCSMc2aMZ1ODm1T_g=@pm.me