Add native target for FreeBSD/riscv.

Add native target for FreeBSD/riscv.

gdb/ChangeLog:

	* Makefile.in (ALLDEPFILES): Add riscv-fbsd-nat.c.
	* NEWS: Mention new FreeBSD/riscv native configuration.
	* configure.host: Add riscv*-*-freebsd*.
	* configure.nat: Likewise.
	* riscv-fbsd-nat.c: New file.

gdb/doc/ChangeLog:

	* gdb.texinfo (Contributors): Add SRI International and University
	of Cambridge for FreeBSD/riscv.