[Support] Define sys::getHostCPUName for RISC-V

The RISCV target doesn't define a "generic" cpu, only "generic-rv32" and
"generic-rv64".  Define sys::getHostCPUName for RISC-V that returns the
correct cpu for the host.

Reviewed By: craig.topper, MaskRay

Differential Revision: https://reviews.llvm.org/D105274