bpf: add extra information for external symbol definitions

bpf: add extra information for external symbol definitions

Extend struct rte_bpf_xsym with new fields to provide information about:
 - for variables - type and size
 - for functions - number of arguments and type/size of each argument
   and return value

Such information would allow validate code to perform
more extensive checking on input BPF program and catch
misbehaving BPF code.

That change would cause ABI/API breakage for librte_bpf.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>