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

[Kito Cheng via cfe-commits]

kito-cheng wrote:

short version: GCC isn't change. 
long version: GCC's configure script isn't change, it's configure script in riscv-gnu-toolchain

But I don't have strong opinion on this change since I believe user should explicitly specify that, otherwise it's really to screw up to select multi-lib or portability issue between different compilers.

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