test: Add a simple test to detected warnings with uint64_t, uintptr_t

These types are problematic because they are typically declared in a
non-standard way in U-Boot. For example, U-Boot uses 'long long' for
int64_t even on a 64-bit machine whereas stdint.h uses 'long'.
Similarly, U-Boot always uses 'long' for intptr_t whereas stdint.h mostly
uses 'int'.

This simple test script runs a few toolchains on a few archs to check for
warnings.

Signed-off-by: Simon Glass <sjg@chromium.org>