RISC-V: Add configure support for riscv*-linux*.

RISC-V: Add configure support for riscv*-linux*.

This adds the target and native configure support, and the NEWS entries for
the new target and native configurations.

	gdb/
	* Makefile.in (ALL_TARGET_OBS): Add riscv-linux-tdep.c.
	(ALLDEPFILES): Add riscv-linux-nat.c, and riscv-linux-tdep.c.
	* NEWS: Mention new GNU/Linux RISC-V target.
	* configure.host: Add riscv*-*-linux*.
	* configure.nat: Add riscv*.
	* configure.tgt: Add riscv*-*-linux*.