This patch makes the binary float macros in real.h use
float_mode_for_size and adds a corresponding decimal_float_mode_for_size
for the decimal macros.

2017-09-05  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* machmode.h (decimal_float_mode_for_size): New function.
	* real.h (REAL_VALUE_TO_TARGET_LONG_DOUBLE): Use float_mode_for_size.
	(REAL_VALUE_TO_TARGET_DOUBLE): Likewise.
	(REAL_VALUE_TO_TARGET_SINGLE): Likewise.
	(REAL_VALUE_TO_TARGET_DECIMAL128): Use decimal_float_mode_for_size.
	(REAL_VALUE_TO_TARGET_DECIMAL64): Likewise.
	(REAL_VALUE_TO_TARGET_DECIMAL32): Likewise.

From-SVN: r251727