Rename RISC-V GCC config directory: riscv64 -> riscv
(to match official RISC-V target for GCC 7.1).

This is only a minimal config required to build c start up (csu).

This fixes build after r322429 ("Make _TO_CPUARCH macro for
ARCH to CPUARCH conversions")

Reported by: lwhsu
Sponsored by: DARPA, AFRL