[clang] [RISCV] Support target attribute for function (PR #65948)

[Piyou Chen via cfe-commits]

BeMg wrote:

This pull request move from https://reviews.llvm.org/D151730.

And update with lastest spec.

1. When it exist the duplicate target attribute, select the lastest one.
2. arch's features will override cpu's features


https://github.com/llvm/llvm-project/pull/65948