Support generating bfloat16 constants. This is a bit awkward, as "DW"
already generates IEEE half precision constants; therefore there is no
longer a single floating-point format for each size. This requires
some replumbing.
Fortunately bfloat16 fits in 64 bits, so support generating them with
a macro that uses __?bfloat16?__() to convert to integers first before
passing them to DW.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
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