[libc] Add strtoint32 and strtoint64 tests

There were regressions in the testing framework due to none of the
functioning buildbots having a 32 bit long. This allowed the 32 bit
version of the strtointeger function to go untested. This patch adds
tests for strtoint32 and strtoint64, which are internal testing
functions that use constant integer sizes. It also fixes the tests to
properly handle these situations.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D151935