bpf: add support for the -mxbpf option

bpf: add support for the -mxbpf option

This patch adds support for a new option -mxbpf.  This tells GCC to
generate code for an expanded version of BPF that relaxes some of the
restrictions imposed by BPF.

2020-05-19  Jose E. Marchesi  <jose.marchesi@oracle.com>

gcc/
	* config/bpf/bpf.opt (mxbpf): New option.
	* doc/invoke.texi (Option Summary): Add -mxbpf.
	(eBPF Options): Document -mxbbpf.