misc/cgo/testcarchive: fix C include path for darwin/arm

misc/cgo/testcarchive: fix C include path for darwin/arm

After CL 22461, c-archive build on darwin/arm is by default compiled
with -shared and installed in pkg/darwin_arm_shared.

Fix build (2nd time...)

Change-Id: Ia2bb09bb6e1ebc9bc74f7570dd80c81d05eaf744
Reviewed-on: https://go-review.googlesource.com/22534
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>