cmd/link: default to internal linking for android/arm64

The bootstrapping process (make.bash) on all other platforms use
internal linking. This change brings android/arm64 in line, fixing the
scary warning on our self-hosted Corellium builders:

warning: unable to find runtime/cgo.a

The linkmode default is changed to internal for all Android programs,
but in practice that won't matter outside our builders: using Go with
Android apps requires buildmode=c-shared which uses linkmode external.

Fixes #31343
Updates #31819

Change-Id: I3b3ada5ed69a7989e6d8e5960bbebf5e1c22aada
Reviewed-on: https://go-review.googlesource.com/c/go/+/207299
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/ld/lib.go

@@ -101,6 +101,7 @@ type Arch struct {
 	Minalign       int
 	Dwarfregsp     int
 	Dwarfreglr     int
+	Androiddynld   string
 	Linuxdynld     string
 	Freebsddynld   string
 	Netbsddynld    string