Remove definition of std::fmaf from libc++. Fixes bug #18910. This function should come from the C standard library. As a drive-by fix, update the tests to remove a warning from -Wabsolute-value

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202990 91177308-0d34-0410-b5e6-96231b3b80d8