[RISCV] Add -mtune support
- The goal of this patch is improve option compatible with RISCV-V GCC, -mcpu support on GCC side will sent patch in next few days.
- -mtune only affect the pipeline model and non-arch/extension related target feature, e.g. instruction fusion; in td file it called TuneFeatures, which is introduced by X86 back-end.
- -mtune accept all valid option for -mcpu and extra alias processor option, e.g. generic, rocket and sifive-7-series, the purpose is option compatible with RISCV-V GCC.
- Processor alias for -mtune will resolve according the current target arch, rv32 or rv64, e.g. rocket will resolve to rocket-rv32 or rocket-rv64.
- Interaction between -mcpu and -mtune:
- -mtune has higher priority than -mcpu for pipeline model and TuneFeatures.
Reviewed By: luismarques
Differential Revision: https://reviews.llvm.org/D89025