[libunwind][CMake] Fix name of LIBUNWIND_SUPPORTS_NODEFAULTLIBS_FLAG

[psamolysov]

Summary

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.

[Ericson2314]

Thanks for catching! I suspect this might explain why some of the refactors @phosek and I were doing before did not work.

[psamolysov]

@Ericson2314 Thank you for the comment. I'm going to land this small patch in a day if there will be no objections.

[mstorsjo]

LGTM. It looks like this is my mistake from D110005.

libunwind/cmake/config-ix.cmake

 # required during compilation (which has the -nostdlib++ or -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
 
 llvm_check_compiler_linker_flag(C "-nostdlib++" CXX_SUPPORTS_NOSTDLIBXX_FLAG)
 if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
 else()
-  llvm_check_compiler_linker_flag(C "-nodefaultlibs" LIBUNWIND_SUPPORTS_NODEFAULTLIBS_FLAG)
+  llvm_check_compiler_linker_flag(C "-nodefaultlibs" C_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (C_SUPPORTS_NODEFAULTLIBS_FLAG)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
   endif()
 endif()
 
 if (CXX_SUPPORTS_NOSTDLIBXX_FLAG OR C_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (LIBUNWIND_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)