[RISCV] Add FallbackRegAltNameIndex to ABIRegAltName.

Remove now redundant fake ABI names from vector registers.

This also fixes a crash that occurs if you use fflags as an instruction
operand in the assembly and use -debug. It's not a valid register
for any instruction since this wouldn't be common. It doesn't have
an ABI name so it crashes the register printing in the debug output.