[RISCV] Support 'generic' as a valid CPU name.

Most other targets support 'generic', but RISCV issues an error.
This can require a special case in tools that use LLVM that aren't
clang.

This patch treats "generic" the same as an empty string and remaps
it to generic-rv/rv64 based on the triple. Unfortunately, it has to
be added to RISCV.td because MCSubtargetInfo is constructed and
parses the CPU before RISCVSubtarget's constructor gets a chance
to remap it. The CPU will then reparsed and the state in the
MCSubtargetInfo subclass will be updated again.

Fixes PR54146.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D121149