configure: only use -ffloat-store on i386

configure: only use -ffloat-store on i386

this option is expensive and only used on old gcc's that lack
-fexcess-precision=standed, but it's not needed on non-i386 archs
where floating point does not have excess precision anyway.

if musl ever supports m68k, i think it will need to be special-cased
too. i'm not aware of any other archs with excess precision.