[libc] fix strtointeger behavior on max values

Previously the check to just return MAX or MIN used the caclulated
number being the maximum absolute value. This was right in every case
except for an unsigned conversion being passed its maximum value with a
negative sign on the front. This should return -MAX, but was returning
just MAX.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D147171