bpf: add s390 testing documentation

This commits adds a document that explains how to test BPF in an s390 QEMU guest.

---

I've been asked a few times how to test BPF on s390, so I took some time to compile from various sources and test the attached guide.

Known limitations:

- Debian host or debootstrap-based chroot is required. Compiling for s390 on other popular distros is much more tedious.
- When using a little-endian host, BTF must be disabled. pahole doesn't seem to support cross-compilation right now.

Documentation/bpf/index.rst | 9 ++

 Documentation/bpf/s390.rst  | 205 ++++++++++++++++++++++++++++++++++++
 2 files changed, 214 insertions(+)
 create mode 100644 Documentation/bpf/s390.rst

index 801a6ed3f2e5..4f5410b61441 100644
@@ -47,6 +47,15 @@ Program types
    prog_flow_dissector
 
 
+Testing BPF
+===========
+
+.. toctree::
+   :maxdepth: 1
+
+   s390
+
+
 .. Links:
 .. _Documentation/networking/filter.txt: ../networking/filter.txt
 .. _man-pages: https://www.kernel.org/doc/man-pages/

2.23.0

Applied, thanks!