[libc] add fuzz target for strtointeger functions

The string to integer conversion functions are well suited to
differential fuzzing, and this patch adds a target to enable just that.
It also fixes a bug in the fuzzing comparison logic and changes atoi
slightly to match the behavior described in the C standard.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D140178