Revert "[OpenMP][Clang] Support for target math functions"

This commit appears to be breaking stage-2 builds on GreenDragon. The
OpenMP wrappers for cmath and math.h are copied into the root of the
resource directory and cause a cyclic dependency in module 'Darwin':
Darwin -> std -> Darwin. This blows up when CMake is testing for modules
support and breaks all stage 2 module builds, including the ThinLTO bot
and all LLDB bots.

CMake Error at cmake/modules/HandleLLVMOptions.cmake:497 (message):
  LLVM_ENABLE_MODULES is not supported by this compiler

llvm-svn: 360192