bpf: Sync linux/bpf.h to tools/

Newly added program, context type and helper is used by tests in a
subsequent patch. Synchronize the header file.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200717103536.397595-12-jakub@cloudflare.com