meson: drop -ffast-math

meson: drop -ffast-math

After reading https://simonbyrne.github.io/notes/fastmath/ I think we
should drop -ffast-math. The JSON code actually looks for NaN, so the
fact it becomes unreliable kinda sucks.

Moreover, we don't do any number crunching. We use floating point fields
only sporadical for trivial math. Hence the optimization is entirely
unnecessary.