nfp: bpf: add helpers for modifying branch addresses

In preparation for better handling of relocations move existing
helper for setting branch offset to nfp_asm.c and add two more.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>