Reapply [compiler-rt] Check for and use -lunwind when linking with -nodefaultlibs (#66584)

If libc++ is available and should be used as the ubsan C++ ABI library,
the check for libc++ might fail if libc++ is a static library, as the
-nodefaultlibs flag inhibits a potential compiler default -lunwind.

Just like the -nodefaultlibs configuration tests for and manually adds a
bunch of compiler default libraries, look for -lunwind too.

This is a reland of #65912.

compiler-rt/cmake/config-ix.cmake

@@ -63,6 +63,16 @@ if (C_SUPPORTS_NODEFAULTLIBS_FLAG)
                         moldname mingwex msvcrt)
     list(APPEND CMAKE_REQUIRED_LIBRARIES ${MINGW_LIBRARIES})
   endif()
+  if (NOT TARGET unwind)
+    # Don't check for a library named unwind, if there's a target with that name within
+    # the same build.
+    check_library_exists(unwind _Unwind_GetRegionStart "" COMPILER_RT_HAS_LIBUNWIND)
+    if (COMPILER_RT_HAS_LIBUNWIND)
+      # If we're omitting default libraries, we might need to manually link in libunwind.
+      # This can affect whether we detect a statically linked libc++ correctly.
+      list(APPEND CMAKE_REQUIRED_LIBRARIES unwind)
+    endif()
+  endif()
 endif ()
 
 # CodeGen options.