runtime: ensure that the module constructor element is RO

The default attribute on the section is read/write.  This would cause
the section to have different attributes when compiled.  The linker
would then preserve the section rather than merge it into the right
location.  This was noticed when linking with `link.exe`.

Author: compnerd

stdlib/public/runtime/SwiftRT-COFF.cpp

@@ -75,6 +75,8 @@ static void swift_image_constructor() {
   swift_addNewDSOImage(&sections);
 }
 
+#pragma section(".CRT$XCIS", long, read)
+
 __declspec(allocate(".CRT$XCIS"))
 extern "C" void (*pSwiftImageConstructor)(void) = &swift_image_constructor;
 #pragma comment(linker, "/include:" STRING(C_LABEL(pSwiftImageConstructor)))