Buildmode c-archive now supports position independent code for
darwin/arm (in addition to darwin/arm64). Make PIC (-shared) the
default for both platforms in the default buildmode.

Without this change, gomobile will go install the standard library
into its separate package directory without PIC support.

Also add -shared to darwin/arm64 in buildmode c-archive, for
symmetry (darwin/arm64 always generates position independent code).

Fixes #15519

Change-Id: If27d2cbea8f40982e14df25da2703cbba572b5c6
Reviewed-on: https://go-review.googlesource.com/22920Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>