use lookup table for malloc bin index instead of float conversion

use lookup table for malloc bin index instead of float conversion

float conversion is slow and big on soft-float targets.

The lookup table increases code size a bit on most hard float targets
(and adds 60byte rodata), performance can be a bit slower because of
position independent data access and cpu internal state dependence
(cache, extra branches), but the overall effect should be minimal
(common, small size allocations should be unaffected).