Restore clang_rt library name on i686-android.

Summary:
Recent changes canonicalized clang_rt library names to refer to
"i386" on all x86 targets. Android historically uses i686.

This change adds a special case to keep i686 in all clang_rt
libraries when targeting Android.

Reviewers: hans, mgorny, beanz

Subscribers: srhines, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D37278

llvm-svn: 312048