[RISCV] Give an explicit error if 'generic' CPU is passed instead of...

[RISCV] Give an explicit error if 'generic' CPU is passed instead of 'generic-rv32' or 'generic-rv64'. Validate 64Bit feature against the triple.

I encountered a project that uses llvm that passes "generic" by
default. While I could fix that project, I wouldn't be surprised
if other projects did something similar. So it seems like
a good idea to provide a better error here.

I've also added validation of the 64Bit feature against the
triple so that we can catch a mismatched CPU before failing in
a mysterious way. We can make it pretty far in isel because we
calculate XLenVT from the triple and use that to set up the legal
integer type.

Reviewed By: luismarques, khchen

Differential Revision: https://reviews.llvm.org/D98307