RISC-V: Fix disassembling Zfinx with -M numeric

This commit fixes floating point operand register names from ABI ones
to dynamically set ones.

gas/ChangeLog:

	* testsuite/gas/riscv/zfinx-dis-numeric.s: Test new behavior of
	Zfinx extension and -M numeric disassembler option.
	* testsuite/gas/riscv/zfinx-dis-numeric.d: Likewise.

opcodes/ChangeLog:

	* riscv-dis.c (riscv_disassemble_insn): Use dynamically set GPR
	names to disassemble Zfinx instructions.