gdb/riscv: Handle passing variadic floating point arguments

gdb/riscv: Handle passing variadic floating point arguments

This commit fixes some test failures in gdb.base/varargs.exp when
running on targets with floating point hardware.  Floating point
unnamed (variadic) arguments should be passed in integer registers
according to the abi.

After this commit I see no failures in gdb.base/varargs.exp on 32 or
64 bit targets with floating point hardware.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_call_arg_scalar_float): Unnamed (variadic)
	arguments are passed in integer registers.
	(riscv_call_arg_complex_float): Likewise.