Also make PIE executables the default build mode, as PIE executables
are required as of Android L.

For #10807

Change-Id: I86b7556b9792105cd2531df1b8f3c8f7a8c5d25c
Reviewed-on: https://go-review.googlesource.com/16055Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>