scripts/bpf: Add syscall commands printer

Add a new target to bpf_doc.py to support generating the list of syscall
commands directly from the UAPI headers. Assuming that developer
submissions keep the main header up to date, this should allow the man
pages to be automatically generated based on the latest API changes
rather than requiring someone to separately go back through the API and
describe each command.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-11-joe@cilium.io