[clang] [RISCV] Enable target attribute when invoked through clang driver (PR #74889)

[Piyou Chen via cfe-commits]

BeMg wrote:

> LGTM
> 
> Related question. If there is an -mcpu on the command line and target attribute changes the march, do we keep the original CPU in the -target-cpu attribute or drop it. The reason for all those negative features from the driver was to make the backend not infer any features from the CPU that weren't in the provided march. So I'm wondering if we have that issue with the target attribute now.

Compiler will keep target-cpu from command line when target attribute doesn't assign new cpu option.

https://github.com/llvm/llvm-project/pull/74889