[BUG]: clang RISC-V `-mcpu` effectively ignored due to default `-march=rv64gc`

### Describe the bug

For "RISC-V rv64gc clang (...)" compilers, using flags such as `-O3 -mcpu=sifive-p670` doesn't result in enabling the given CPU architecture, due to the default `-march=rv64gc` flag.

### Steps to reproduce

See reproduction link

### Expected behavior

The clang compiler sees `clzw` / vector use as gcc does. Can also see the desired behavior in an x86-64 clang compiler manually overridden to do RISC-V via `--target=riscv64-linux-gnu -O3 -mcpu=sifive-p670`.

A potential fix would be to not automatically add the default `-march=rv64gc` flag if `-mcpu` is present. (or perhaps not add the `-march` at all? Pretty sure it's the default anyways)

### Reproduction link

https://riscvc.godbolt.org/z/sWoWxKEn4

### Screenshots

Not applicable

### Operating System

_No response_

### Browser version

_No response_