mysql_client_test_nonblock fails when compiled with clang

mysql_client uses some inline assembly code to switch thread stacks.
This works, however tools that perform backtrace get confused to fix
this we write a specific constant to signify bottom of stack. This
constant is needed when compiling with CLang as well.

mysys/my_context.c

@@ -206,7 +206,8 @@ my_context_spawn(struct my_context *c, void (*f)(void *), void *d)
     (
      "movq %%rsp, (%[save])\n\t"
      "movq %[stack], %%rsp\n\t"
-#if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 4)) && !defined(__INTEL_COMPILER)
+#if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 4) || __clang__) && \
+     !defined(__INTEL_COMPILER)
      /*
        This emits a DWARF DW_CFA_undefined directive to make the return address
        undefined. This indicates that this is the top of the stack frame, and