Use PIE on Android

This is OK to do given:
  - PIE is supported on Android starting with API 16.
  - The bots are running API 18.
  - API < 16 now has a 12.5% market share[0] as of 2015-04-29.

Closes #17437.

[0] https://developer.android.com/about/dashboards/index.html

src/librustc_back/target/arm_linux_androideabi.rs

@@ -13,8 +13,6 @@ use target::Target;
 pub fn target() -> Target {
     let mut base = super::android_base::opts();
     base.features = "+v7".to_string();
-    // FIXME #17437 (and #17448): Android doesn't support position dependent executables anymore.
-    base.position_independent_executables = false;
 
     Target {
         data_layout: "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-\