gdb: support for eBPF

gdb: support for eBPF

This patch adds basic support for the eBPF target: tdep and build
machinery.  The accompanying simulator is introduced in subsequent
patches.

gdb/ChangeLog:

2020-08-04  Weimin Pan <weimin.pan@oracle.com>
	    Jose E. Marchesi  <jose.marchesi@oracle.com>

	* configure.tgt: Add entry for bpf-*-*.
	* Makefile.in (ALL_TARGET_OBS): Add bpf-tdep.o
	(ALLDEPFILES): Add bpf-tdep.c.
	* bpf-tdep.c: New file.
	* MAINTAINERS: Add bpf target and maintainer.

gdb/doc/ChangeLog:

2020-08-04  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* gdb.texinfo (Contributors): Add information for the eBPF
	support.
	(BPF): New section.