[CMake][libunwind] Use -nodefaultlibs for CMake checks

Since libunwind is built with -nodefaultlibs, we should be using this
option even for CMake checks to avoid any inconsistency and also to
avoid dependency on a working C++ standard library just for the setting
up the build itself. The implementation is largely similar to the one
used by libc++.

Differential Revision: https://reviews.llvm.org/D31640

llvm-svn: 299796