config.sub patch for RISC-V

[Palmer Dabbelt]

This patch provides support for the RISC-V ISA: http://riscv.org/

Not yet upstreamed ports of the binutils, GCC, LLVM, glibc, and Linux
exist for RISC-V, and a number of hardware implementations exist --
more more information can be seen at http://riscv.org .  We'd like to
start getting RISC-V recognized by configure so it's easier for people
to start porting stuff.

Thanks!