[RISCV] Add Xsfmm32a shorthand extension. (#181957)
This extension is shorthand for Xsfmm32a8i, Xsfmm32a16f, and
Xsfmm32a32f.
It was mistakenly left out of an earlier version of the public
specification, but is now present. See
https://www.sifive.com/document-file/xsfmm-matrix-extensions-specification
