Conditionalize constant folding of math intrinsics on the availability of an implementation on the host.  This is a little bit unfortunate, but until someone decides to implement a full libm for APFloat, we don't have a better way to get this functionality.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174561 91177308-0d34-0410-b5e6-96231b3b80d8
Fix CMake detection of various cmath functions, and XFAIL the test on platforms that are known to be missing them.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174564 91177308-0d34-0410-b5e6-96231b3b80d8