gdb/riscv: Apply NaN boxing when writing return values into registers

When setting up function parameters we already perform NaN boxing, as
required by the RISC-V ABI, however, we don't do this when writing
values into registers as part of setting up a return value.

This commit moves the NaN boxing code into a small helper function,
and then makes use of this function when setting up function
parameters, and also when setting up return values.

This should resolve this failure:

  FAIL: gdb.base/return-nodebug.exp: float: full width of the returned result

gdb/ChangeLog:

	PR gdb/25489
	* riscv-tdep.c (riscv_arg_info::c_offset): Update comment.
	(riscv_regcache_cooked_write): New function.
	(riscv_push_dummy_call): Use new function.
	(riscv_return_value): Likewise.