[libc] Fix various -Wconversion warnings in strto*l test code.

The Fuchsia build compiles the libc and test code with lots
of warnings enabled, including all the integer conversion warnings.
There was some sloppy type usage here that triggered some of those.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D108800

GitOrigin-RevId: 4e1a164d7bd53653f79decc121afe784d2fde0a7
4 files changed