target/arm: Implement FPCR.EBF=1 semantics for bfdotadd()

target/arm: Implement FPCR.EBF=1 semantics for bfdotadd()

Implement the FPCR.EBF=1 semantics for bfdotadd() operations:
 * is_ebf() sets up fpst and fpst_odd
 * bfdotadd_ebf() implements the fused paired-multiply-and-add
   operation that we need

The paired-multiply-and-add is similar to f16_dotadd() and
we use the same trick here as in that function, but the inputs
here are bfloat16 rather than float16.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>