[libc] fix -Wconversion in float_to_string.h (#74369)

Fixes:
libc/src/__support/float_to_string.h:551:48: error: conversion from
‘long
unsigned int’ to ‘int32_t’ {aka ‘int’} may change value
[-Werror=conversion]
551 | const int32_t shift_amount = SHIFT_CONST + (-exponent - IDX_SIZE *
idx);
| ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Observed in gcc fullbuilds.

IDX_SIZE is a size_t (aka 'long unsigned int'), but has the value 128,
so the
expression is undergoing implicit promotion.

Link: https://lab.llvm.org/buildbot/#/builders/250/builds/14891