[Analysis] Disable exp/exp2/pow finite lib calls on Android with -ffast-math.

Summary:
Since r322087, glibc's finite lib calls are generated when possible.
However, glibc is not supported on Android. Therefore this change
enables llvm to finely distinguish between linux and Android for
unsupported library calls. The change also include some regression
tests.

Reviewers: srhines, pirama

Reviewed By: srhines

Subscribers: kongyi, chh, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D42288

llvm-svn: 323187