Link Linux/Android executables with -pie for position independence

PIE is supported in Android 4.1.0 and later (API 16), and is required by
Android 5.0.0 (API 21) and later.

BUG=crashpad:30

Change-Id: I8b08d970ede0e33cba41f81e5ed6cb0dc88bc7c9
Reviewed-on: https://chromium-review.googlesource.com/405447
Reviewed-by: Robert Sesek <rsesek@chromium.org>