[RISCV][Driver] Allow the use of CPUs with a different XLEN than the triple.

Our downstream toolchain release got some issue is we set the default
triple by creating symbolic link of clang like `riscv64-unknown-elf-clang`,
and has lots of multi-lib config including rv32's config.

However when we trying to set arch by a 32 bit CPU like generic-rv32
but got error message below:
error: unsupported argument 'generic-rv32' to option '-mcpu='

`generic-rv32` is listed in the output of `-mcpu=help`, that
might be confusing for user since help message say supported.

So let clang driver also consider -mcpu option during computing
the target triple to archvie that.

Reviewed By: asb, craig.topper

Differential Revision: https://reviews.llvm.org/D148124