[RISCV] Add MC layer support for XSfmm*. (#133031)
This adds assembler/disassembler support for XSfmmbase 0.6 and related
SiFive matrix multiplication extensions based on the spec here
https://www.sifive.com/document-file/xsfmm-matrix-extensions-specification
Functionality-wise, this is the same as the Zvma extension proposal that
SiFive shared with the Attached Matrix Extension Task Group. The
extension names and instruction mnemonics have been changed to use
vendor prefixes.
Note this is a non-conforming extension as the opcodes used here are in
the standard opcode space in OP-V or OP-VE.
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Co-authored-by: Brandon Wu <brandon.wu@sifive.com>
