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

[Craig Topper via cfe-commits]

https://github.com/topperc approved this pull request.

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.

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