[RISCV] Add codegen support for RISCV XTHeadVdot Extension

[JojoR]

Summary

This patch adds codegen support for XTHeadVdot extension,
which is documented here, it's based on standard vector extension v1.0:

https://github.com/T-head-Semi/thead-extension-spec

[craig.topper]

llvm/include/llvm/IR/IntrinsicsRISCVXTHead.td

5

Use DefaultAttrsIntrinsic

[JojoR]

@craig.topper @kito.cheng @asb @eopXD @jrtc27
Any other suggestions ?

[craig.topper]

LGTM