Make sure that REAL_MODE_FORMAT aborts if it is passed an invalid mode,
rather than stepping beyond the bounds of an array.  It turned out that
some code was passing non-float modes to the real.h routines.

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

gcc/
	* real.h (REAL_MODE_FORMAT): Abort if the mode isn't a
	SCALAR_FLOAT_MODE_P.

From-SVN: r229579