RISC-V: xtheadfmemidx: Use fp register in mnemonics

RISC-V: xtheadfmemidx: Use fp register in mnemonics

Although the encoding for scalar and fp registers is identical,
we should follow common pratice and use fp register names
when referencing fp registers.

The xtheadmemidx extension consists of indirect load/store instructions
which all load to or store from fp registers.
Let's use fp register names in this case and adjust the test cases
accordingly.

gas/
    * testsuite/gas/riscv/x-thead-fmemidx-fail.l: Updated since rd need to
    be float register.
    * testsuite/gas/riscv/x-thead-fmemidx-fail.s: Likewise.
    * testsuite/gas/riscv/x-thead-fmemidx.d: Likewise.
    * testsuite/gas/riscv/x-thead-fmemidx.s: Likewise.
opcodes/
    * riscv-opc.c (riscv_opcodes): Updated since rd need to be float register.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>