[clang] [llvm] [clang][RISCV] Change default abi when only have f extension but no d extension (PR #73489)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> when only have f extension but no d extension

This can be simplified to "w/ f extension but w/o d extension"

https://github.com/llvm/llvm-project/pull/73489