Add a generic config and auto detection for riscv64 targest

[imciner2]

In Julia, we compile a single library for BLIS that targets our RISC-V build (we only build for 64-bit RISC-V). Looking at the BLIS targets, there are several, including specialized ones for SiFive processors:

blis/config_registry

Line 58 in e23d936

# RISC-V architectures.

. We would like to support as much as we can, so it would be great if a meta target for riscv64 could be added that would include all the 64-bit RISC-V targets, and have auto dispatch to the target at runtime. For now, we are just building with generic on riscv64.