cmd/cgo: make JNI's jobject type map to uintptr in Go

cmd/cgo: make JNI's jobject type map to uintptr in Go

The jobject type is declared as a pointer, but some JVMs
(Dalvik, ART) store non-pointer values in them. In Go, we must
use uintptr instead of a real pointer for these types.

This is similar to the CoreFoundation types on Darwin which
were "fixed" in CL 66332.

Update #22906
Update #21897

RELNOTE=yes

Change-Id: I0d4c664501d89a696c2fb037c995503caabf8911
Reviewed-on: https://go-review.googlesource.com/81876
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>