bpf: JIT helpers for fmod_ret progs

* Split the invoke_bpf program to prepare for special handling of
  fmod_ret programs introduced in a subsequent patch.
* Move the definition of emit_cond_near_jump and emit_nops as they are
  needed for fmod_ret.
* Refactor branch target alignment into its own generic helper function
  i.e. emit_align.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200304191853.1529-3-kpsingh@chromium.org