Enable -pie and --enable-new-dtags by default on Android.

Summary:
Also enable -no-pie on Gnu toolchain (previously available on Darwin only).

Non-PIE executables won't even start on recent Android, and DT_RPATH is ignored by the loader.

Reviewers: srhines, danalbert

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38430

llvm-svn: 316606