RISC-V: replace cbom instructions with an insn-def

RISC-V: replace cbom instructions with an insn-def

Using the cbom instructions directly in ALT_CMO_OP, requires toolchain
support for the instructions. Using an insn-def will allow for removal
of toolchain version checks in the build system & simplification of the
proposed [1] function-based CMO scheme.

Link: https://lore.kernel.org/linux-riscv/[email protected]/
Signed-off-by: Conor Dooley <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>