Fix CPU-identification macros for RISC-V.

Fix CPU-identification macros for RISC-V.

Turns out that RISC-V intentionally doesn't follow the common
naming pattern for CPU-identification macros.  But the point of
2ef57e636 is to have a common pattern, so we're going to override
their opinion.

Discussion: https://postgr.es/m/CA+hUKGL8Hs-phHPugrWM=5dAkcT897rXyazYzLw-Szxnzgx-rA@mail.gmail.com