floatformat.h: Add bfloat16 support.

floatformat.h: Add bfloat16 support.

This change is motivated by a patchset that adds bfloat16 debugging
support for new avx512 instructions to GDB. The gdb thread can be found
here: https://sourceware.org/pipermail/gdb-patches/2020-July/170820.html

include:
2020-08-17  Felix Willgerodt  <felix.willgerodt@intel.com>

	* floatformat.h (floatformat_bfloat16_big): New.
	(floatformat_bfloat16_little): New.

libiberty:
2020-08-17  Felix Willgerodt  <felix.willgerodt@intel.com>

	* floatformat.c (floatformat_bfloat16_big): New.
	(floatformat_bfloat16_little): New.