Add __dso_handle to bootloader and kernel

Clang references these when building for x86_64.

src/system/boot/loader/main.cpp

@@ -26,6 +26,9 @@
 #endif
 
 
+void *__dso_handle;
+
+
 extern "C" int
 main(stage2_args *args)
 {

src/system/kernel/main.cpp

@@ -65,6 +65,9 @@
 #	define TRACE(x...) ;
 #endif
 
+
+void *__dso_handle;
+
 bool gKernelStartup = true;
 bool gKernelShutdown = false;