Currently, cgo converts integer macros into int64 if it's possible.
As a result, some macros which satisfy

math.MaxInt64 < x <= math.MaxUint64

will lose their original values.

This CL introduces the new probe to check signs,
so we can handle signed ints and unsigned ints separately.

Fixes #20369

Change-Id: I002ba452a82514b3a87440960473676f842cc9ee
Reviewed-on: https://go-review.googlesource.com/43476Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>