RISC-V: NaN-box FP values smaller than an FP register.

RISC-V: NaN-box FP values smaller than an FP register.

The hardware requires that values in FP registers be NaN-boxed, so we must
extend them with 1's instead of 0's as we do for integer values.

	gdb/
	* riscv-tdep.c (riscv_push_dummy_call) <in_reg>: Check for value in
	FP reg smaller than FP reg size, and fill with -1 instead of 0.