RISC-V: Add testcases for z[fdq]inx

Use gpr when the zfinx enable, the testcases contain float
instructions that reuse by z[fdq]inx.

gas/ChangeLog:

* testsuite/gas/riscv/zdinx.d: New test.
* testsuite/gas/riscv/zdinx.s: New test.
* testsuite/gas/riscv/zfinx.d: New test.
* testsuite/gas/riscv/zfinx.s: New test.
* testsuite/gas/riscv/zqinx.d: New test.
* testsuite/gas/riscv/zqinx.s: New test.

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>