[clang] [llvm] [RISCV] Add SpacemiT XSMTVDot (SpacemiT Vector Dot Product) extension. (PR #151706)

[via llvm-commits]

================
@@ -531,6 +531,9 @@ The current vendor extensions supported are:
 ``XAndesVDot``
   LLVM implements `version 5.0.0 of the Andes Vector Dot Product Extension specification <https://github.com/andestech/andes-v5-isa/releases/download/ast-v5_4_0-release/AndeStar_V5_ISA_Spec_UM165-v1.5.08-20250317.pdf>`__ by Andes Technology. All instructions are prefixed with `nds.` as described in the specification.
 
+``XSMTVDot``
+  LLVM implements `version 1.0.0 of the SpacemiT Vector Dot Product Extension specification <https://developer.spacemit.com/documentation?token=BWbGwbx7liGW21kq9lucSA6Vnpb#2.1>`__ by SpacemiT. All instructions are prefixed with `smt.` as described in the specification.
----------------
link-xyq wrote:

Frankly, we're not quite sure how to express this meaning clearly and concisely.
Do you mean to change it to the following statement?
```
``XSMTVDot``
  LLVM implements `version 1.0.0 of the SpacemiT Vector Dot Product Extension specification <https://github.com/space-mit/riscv-ime-extension-spec/releases/tag/v0429>`__ by SpacemiT.  The X60 processor supports a subset of these instructions; All instructions are prefixed with `smt.` as described in the specification.
```


https://github.com/llvm/llvm-project/pull/151706