Fix real_2expN mode arguments in fixed-value.c

Fix real_2expN mode arguments in fixed-value.c

fixed-value.c was passing a fixed-point mode to the floating-point
real_2expN routine.  That didn't cause a problem in practice because
all real_2expN did was check for decimal float modes, but it triggered
a failure with an upcoming patch.

Tested on x86_64-linux-gnu, arm-linux-gnueabi and aarch64-linux-gnu.

gcc/
	* fixed-value.c (check_real_for_fixed_mode, fixed_from_string)
	(fixed_to_decimal, fixed_convert_from_real)
	(real_convert_from_fixed): Fix mode arguments to real_2expN.

From-SVN: r229580