fpu: Add overflow_raises_invalid to FloatFmt

ARM Alt HP raises different exceptions on overflow than is
standard for IEEE when saturating a value.  Add a flag to
control this effect.

Reviewed-by: Chao Liu <chao.liu.zevorn@gmail.com>
Signed-off-by: Max Chou <max.chou@sifive.com>
[rth: Split out of a larger patch]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>