[Driver] Default to `-z now` and `-z relro` on Android.

Summary:
RTLD_LAZY is not supported on Android (though failing to use `-z now`
will work since it is assumed by the loader).

RelRO is required.

Reviewers: srhines, pirama

Reviewed By: srhines

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53117

llvm-svn: 344295