[clang] [llvm] [Driver][RISCV] Support the new `-mtune` syntax with tuning feature string (PR #196653)

[Craig Topper via cfe-commits]

================
@@ -203,6 +203,7 @@ Makes programs 10x faster by doing Special New Thing.
 * Support for the experimental `XRivosVisni` vendor extension has been removed.
 * Adds experimental assembler support for the 'Zvvmm` (RISC-V Integer Matrix Multiply-Accumulate) extension.
 * Adds support for 'Ziccid' (Instruction/Data Coherence and Consistency) extension.
+* A new `-mtune` syntax that supports processor-specific tuning feature string is added.
----------------
topperc wrote:

This belongs in the clang release notes.

https://github.com/llvm/llvm-project/pull/196653