[libunwind][CMake] Fix name of LIBUNWIND_SUPPORTS_NODEFAULTLIBS_FLAG

The CMake variable LIBUNWIND_SUPPORTS_NODEFAULTLIBS_FLAG has been renamed
into C_SUPPORTS_NODEFAULTLIBS_FLAG because the last one is used in the
confix-ix.cmake file while the variable with the original name is not used
at al.

Differential Revision: https://reviews.llvm.org/D126466