selftests/bpf: Build each test runner instance in its own sub-make

Replace DEFINE_TEST_RUNNER/DEFINE_TEST_RUNNER_RULES with
Makefile.runner, invoked once per test runner instance. Each invocation
us

selftests/bpf: Build each test runner instance in its own sub-make

Replace DEFINE_TEST_RUNNER/DEFINE_TEST_RUNNER_RULES with
Makefile.runner, invoked once per test runner instance. Each invocation
uses ordinary make rules in a single-flavor namespace.

The main Makefile owns shared build outputs and the kselftest
run/install rules. Makefile.skel supplies the BPF object and skeleton
rules used by both the main Makefile and the runners. The main Makefile
builds the default flavor before invoking unflavored runners; flavored
runners build in separate directories.

Build shared userspace objects once and link them into every flavor.
Order their compilation after the bpftool sub-build, which installs the
libbpf-internal headers they include. Pass the assembled CFLAGS and
LDFLAGS to runners on their command lines.

Build and copy runtime fixtures alongside runner compilation so module
builds do not delay runner startup. They are prerequisites of the
test_progs, test_progs-<flavor> and all goals rather than of the runner
binaries, so a goal spelled as a binary's path builds the binary alone.
Installation copies the default flavor's BPF objects, preserving the
previous result.

Generate prog_tests/tests.h and map_tests/tests.h through ordinary
recipes, avoiding generation during make -n. Signed skeletons explicitly
depend on the private key. The flavored runners' objects and the test
objects are targets of the sub-makes only, and the bare linked-object
names (make linked_funcs1.bpf.o) are no longer targets. Build-log
details change: skeleton messages adopt the common format and output
stream, the shared objects log as CC, the fixture copy prints no
EXT-COPY line and TEST-HDR carries no runner tag.

Co-developed-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Mykola Lysenko <nickolay.lysenko@gmail.com>
Link: https://lore.kernel.org/bpf/20260921075855.2065871-10-nickolay.lysenko@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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