[libc] Prevent system headers from being included for the GPU build

It's very important that the GPU build does not include any system
directories. We currently use `-ffreestanding` to disable a lot of these
features, but we can still accidentally include them if they are not
provided by `libc` yet. This patch changes this to use `-nostdinc` to
disable all standard search paths. Then we use the compiler's resource
directory to pick up the provided headers like `stdint.h`.

Differential Revision: https://reviews.llvm.org/D159445