[binutils-gdb] RISC-V: Add native linux support.

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 3c77f97eb0825f5808f4a3a01aa5cc2fcffd5ed4 ***

Author: Jim Wilson <jimw@sifive.com>
Branch: master
Commit: 3c77f97eb0825f5808f4a3a01aa5cc2fcffd5ed4

RISC-V: Add native linux support.

Add initial native support for riscv*-linux*.

	gdb/
	* riscv-linux-nat.c: New file.