cmd/gomobile: build android stdlib as PIC

Fixes golang/go#12581

Change-Id: I9e462b76037f0f89f1d3e59b9aef899d21de925a
Reviewed-on: https://go-review.googlesource.com/15803
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/cmd/gomobile/init.go b/cmd/gomobile/init.go
index 3961934..9e2adcb 100644
--- a/cmd/gomobile/init.go
+++ b/cmd/gomobile/init.go
@@ -145,7 +145,7 @@
 
 	// Install standard libraries for cross compilers.
 	start := time.Now()
-	if err := installStd(androidArmEnv); err != nil {
+	if err := installStd(androidArmEnv, "-buildmode=c-shared"); err != nil {
 		return err
 	}
 	if err := installDarwin(); err != nil {