tools, bpf_asm: Warn when jumps are out of range

When compiling larger programs with bpf_asm, it's possible to
accidentally exceed jt/jf range, in which case it won't complain, but
rather silently emit a truncated offset, leading to a "happy debugging"
situation.

Add a warning to help detecting such issues. It could be made an error
instead, but this might break compilation of existing code (which might
be working by accident).

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191107100349.88976-1-iii@linux.ibm.com