doc: BPF CO-RE documentation

doc: BPF CO-RE documentation

Document the new command line options (-mco-re and -mno-co-re), the new
BPF target builtin (__builtin_preserve_access_index), and the new BPF
target attribute (preserve_access_index) introduced with BPF CO-RE.

gcc/ChangeLog:

	* doc/extend.texi (BPF Type Attributes) New node.
	Document new preserve_access_index attribute.
	Document new preserve_access_index builtin.
	* doc/invoke.texi: Document -mco-re and -mno-co-re options.