[libc][NFC] make atoi undefined cases match std

[libc][NFC] make atoi undefined cases match std

The standard describes atoi as:

"equivalent to atoi: (int)strtol(nptr, (char **)NULL, 10)"

Previously, our behavior was slightly different on numbers larger than
INT_MAX, but this patch changes it to just do the cast instead. Both of
these are valid since the standard says

"If the value of the result cannot be represented, the
behavior is undefined."

But matching existing behavior makes differential fuzzing easier.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D140350