RISC-V: Print FP regs as union of float types.

RISC-V: Print FP regs as union of float types.

A 64-bit FP register can hold either a single or double float value, so
print it as both types by using a union type for FP registers.  Likewise
for 128-bit regs which can also hold long double.

	gdb/
	* riscv-tdep.c (riscv_fpreg_d_type, riscv_fpreg_q_type): New.
	(riscv_register_type): Use them.
	(riscv_print_one_register_info): Handle union of floats same as float.
	* riscv-tdep.h (struct gdbarch_tdep): Add riscv_fpreg_d_type and
	riscv_fpreg_q_type fields.