[AArch64] Add more complete support for BF16

We can use a small amount of integer arithmetic to round FP32 to BF16
and extend BF16 to FP32.

While a number of operations still require promotion, this can be
reduced for some rather simple operations like abs, copysign, fneg but
these can be done in a follow-up.

A few neat optimizations are implemented:
- round-inexact-to-odd is used for F64 to BF16 rounding.
- quieting signaling NaNs for f32 -> bf16 tries to detect if a prior
  operation makes it unnecessary.