[CodeGen] Mark fma as const for Android

Summary:
r318093 sets fma, fmaf, fmal as const for Gnu and MSVC.  Android also
does not set errno for these functions.  So mark these const for
Android.

Reviewers: spatel, efriedma, srhines, chh, enh

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D44852

Change-Id: Ifbee65d27f12691703858c3784271fce02f20ea1
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@328552 91177308-0d34-0410-b5e6-96231b3b80d8