Add target hook CodeGen queries when generating builtin pow*.

Add target hook CodeGen queries when generating builtin pow*.

Without fmath-errno, Clang currently generates calls to @llvm.pow.* intrinsics
when it sees pow*(). This may not be suitable for all targets (for
example le32/PNaCl), so the attached patch adds a target hook that CodeGen
queries. The target can state its preference for having or not having the
intrinsic generated. Non-PNaCl behavior remains unchanged;
PNaCl-specific test added.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185568 91177308-0d34-0410-b5e6-96231b3b80d8