RISC-V: Fix missing instruction classes in error messages

Add 6 missing instruction class cases to riscv_multi_subset_supports_ext()
to provide proper extension names in error messages instead of producing
"internal: unreachable INSN_CLASS_*" errors.

These instruction classes exist in riscv_multi_subset_supports() but were
missing from riscv_multi_subset_supports_ext(), causing the assembler to
produce internal errors when instructions are used without the required
-march specification.

Missing classes added:
- INSN_CLASS_ZABHA_AND_ZACAS (zabha and zacas)
- INSN_CLASS_XVENTANACONDOPS (xventanacondops)
- INSN_CLASS_XSFVCP (xsfvcp)
- INSN_CLASS_XSFVQMACCQOQ (xsfvqmaccqoq)
- INSN_CLASS_XSFVQMACCDOD (xsfvqmaccdod)
- INSN_CLASS_XSFVFNRCLIPXFQF (xsfvfnrclipxfqf)

Before this fix:
  Error: internal: unreachable INSN_CLASS_*
  Error: unrecognized opcode `amocas.b a0,a1,(a2)'

After this fix:
  Error: unrecognized opcode `amocas.b a0,a1,(a2)', extension `zabha' and `zacas' required

bfd/
    * elfxx-riscv.c (riscv_multi_subset_supports_ext): Add 6
    missing instruction class cases.

Signed-off-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com>