[libc] Fix the wrapper headers for 'toupper' and 'tolower'

Summary:
The GNU headers like to reassign this function to a new function which
the optimizer will pick up unless compiling with `O0`. This uses an
external LUT which we don't have and fails to link. This patch makes
sure that the GPU portion does not include these extra definitions and
we only use the ones we support. It's hacky, but it's the only way to
disable it.