Revert "[Libomptarget] Use freestanding stdint.h header for DeviceRTL"

This patch breaks the handling of `printf` in the OpenMP library. Usiing
`-ffreestanding` prevents clang from emitting LLVM builtins, which we
use for OpenMP printing support. Shelve this until we have functioning
`printf` in the GPU `libc` and we can remove that code.

This reverts commit a92eaa3e.