unwind: clean up warnings from the build

Now thta the build is split, clean up some of the warnings in the build:
  cc1: warning: command line option '-nostdinc++' is valid for C++/ObjC++ but not for C
  cc1: warning: command line option '-fno-rtti' is valid for C++/ObjC++ but not for C

Append the C++ specific flags specifically to the C++ sources.  Avoids the
spurious warnings due to invalid flags being passed during the compilation of
C++ sources.

llvm-svn: 235797