Add two simple example BPF schedulers - simple and qmap.

* simple: In terms of scheduling, it behaves identical to not having any
  operation implemented at all. The two operations it implements are only to
  improve visibility and exit handling. On certain homogeneous
  configurations, this actually can perform pretty well.

* qmap: A fixed five level priority scheduler to demonstrate queueing PIDs
  on BPF maps for scheduling. While not very practical, this is useful as a
  simple example and will be used to demonstrate different features.

v7: - Compat helpers stripped out in prepartion of upstreaming as the
      upstreamed patchset will be the baselinfe. Utility macros that can be
      used to implement compat features are kept.

    - Explicitly disable map autoattach on struct_ops to avoid trying to
      attach twice while maintaining compatbility with older libbpf.

v6: - Common header files reorganized and cleaned up. Compat helpers are
      added to demonstrate how schedulers can maintain backward
      compatibility with older kernels while making use of newly added
      features.

    - simple_select_cpu() added to keep track of the number of local
      dispatches. This is needed because the default ops.select_cpu()
      implementation is updated to dispatch directly and won't call
      ops.enqueue().

    - Updated to reflect the sched_ext API changes. Switching all tasks is
      the default behavior now and scx_qmap supports partial switching when
      `-p` is specified.

    - tools/sched_ext/Kconfig dropped. This will be included in the doc
      instead.

v5: - Improve Makefile. Build artifects are now collected into a separate
      dir which change be changed. Install and help targets are added and
      clean actually cleans everything.

    - MEMBER_VPTR() improved to improve access to structs. ARRAY_ELEM_PTR()
      and RESIZEABLE_ARRAY() are added to support resizable arrays in .bss.

    - Add scx_common.h which provides common utilities to user code such as
      SCX_BUG[_ON]() and RESIZE_ARRAY().

    - Use SCX_BUG[_ON]() to simplify error handling.

v4: - Dropped _example prefix from scheduler names.

v3: - Rename scx_example_dummy to scx_example_simple and restructure a bit
      to ease later additions. Comment updates.

    - Added declarations for BPF inline iterators. In the future, hopefully,
      these will be consolidated into a generic BPF header so that they
      don't need to be replicated here.

v2: - Updated with the generic BPF cpumask helpers.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>