[Libomptarget] Use freestanding stdint.h header for DeviceRTL

The `stdint.h` header provides the standard types. Previously we used
`-nostdinc` and defined these ourselves. This patch switches to a
freestanding version which should work properly. Without
`-ffreestanding` the `stdint.h` header will include other libraries. But
in a freestanding environment it should work given the primitives.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D145963