[clang] [llvm] [RISCV][FMV] Support target_clones (PR #85786)

[Piyou Chen via cfe-commits]

BeMg wrote:

Update:

1. Remove all the `__riscv_feature_bits.length` checker due to unnecessary now.
2. Rebase with origin/main for tablegen change
3. Extract some common code as helper function
4. Replace RequireFeautreBits with getExtensionBitmask and compute the concrete bitmask inside CodeGenFunction::EmitRISCVCpuSupports
5. If there are extensions without bitmask be used, report the warning during Sema part.  

https://github.com/llvm/llvm-project/pull/85786