[CMake] Rename back SIMPLE_SOURCE to compile as C++

This was changed in rL276151 and causes problems if the C++ compiler does not
support the same arches as the C compiler.
For the builtins, only the C compiler is tested in try_compile_only.

Additionally, -fno-exceptions is passed in (if available) to work around
the case where no libunwind is available.

Differential Revision: https://reviews.llvm.org/D23654

llvm-svn: 282054