[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

llvm-svn: 328552