Add ARM-specific Bfloat format support to middle-end

2019-12-11  Stam Markianos-Wright  <stam.markianos-wright@arm.com>

	* real.c (struct arm_bfloat_half_format,
	encode_arm_bfloat_half, decode_arm_bfloat_half): New.
	* real.h (arm_bfloat_half_format): New.

From-SVN: r279216