selftests/bpf: Add tests for wakeup_sources kfuncs

Introduce a set of BPF selftests to verify the safety and functionality
of wakeup_source kfuncs.

The suite includes:
1. A functional test (test_wakeup_source.c) that iterates over the
   global wakeup_sources list. It uses CO-RE to read timing statistics
   and validates them in user-space via the BPF ring buffer.
2. A negative test suite (wakeup_source_fail.c) ensuring the BPF
   verifier correctly enforces reference tracking and type safety.
3. Enable CONFIG_PM_WAKELOCKS in the test config, allowing creation of
   wakeup sources via /sys/power/wake_lock.

A shared header (wakeup_source.h) is introduced to ensure consistent
memory layout for the Ring Buffer data between BPF and user-space.

Signed-off-by: Samuel Wu <wusamuel@google.com>
Link: https://lore.kernel.org/r/20260511174559.659782-3-wusamuel@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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