cpp: mark __dso_handle as weak.

The POSIX target uses the native crt0 which already provides
__dso_handle.

Signed-off-by: Michael Hope <mlhx@google.com>
diff --git a/subsys/cpp/cpp_dtors.c b/subsys/cpp/cpp_dtors.c
index 2e6f462..b4017e5 100644
--- a/subsys/cpp/cpp_dtors.c
+++ b/subsys/cpp/cpp_dtors.c
@@ -12,7 +12,7 @@
 
 #include <toolchain.h>
 
-void *__dso_handle = 0;
+__weak void *__dso_handle;
 
 /**
  * @brief Register destructor for a global object