Demos: Add a dynamic linking demo to show off dlfcn methods

The LinkDemo program calls dlopen/dlsym/dlclose to try and load
a dyanmic library from /usr/lib. It read a global variable and
calls a global function (extern "C" of course :) ).

There a few hacks left in the LinkLib dynamic library, however.
In order to get the linker to stop complaining, we have to use
-nostartfiles -ffreestanding otherwise it will link crt0.o to our
shared object, which is definitely not right as the _init function
for a main program (that calls main) is not suitable for our lib

Demos/DynamicLink/LinkLib/DynamicLib.cpp

@@ -0,0 +1,72 @@
+#include <AK/kstdio.h>
+#include <AK/String.h>
+#include <assert.h>
+#include <stdio.h>
+
+// FIXME: See Makefile. We need -ffreestanding and -nostartfiles to
+//     Get GCC to stop linking crt0.o w/our .so.
+//     So, we need __dso_handle. ... Yikes
+extern void* __dso_handle __attribute__((__section__(".sdata")));
+extern void* __dso_handle __attribute__((__visibility__("hidden")));
+void* __dso_handle = (void*)1234; // FIXME: Is the dynamic linker supposed to set this value?
+
+// FIXME: Things defined in crt0 >:(
+__thread int errno;
+char* __static_environ[] = { nullptr }; // We don't get the environment without some libc workarounds..
+char** environ = __static_environ;
+bool __environ_is_malloced = false;
+extern unsigned __stack_chk_guard;
+unsigned __stack_chk_guard = (unsigned)0xc0000c13;
+
+[[noreturn]] void __stack_chk_fail()
+{
+    ASSERT_NOT_REACHED();
+}
+
+// FIXME: Because we need to call printf, and we don't have access to the stout file descriptor
+//     from the main executable, we need to create our own copy in __stdio_init :/
+//     Same deal for malloc init. We're essentially manually calling __libc_init here.
+extern "C" void __stdio_init();
+extern "C" void __malloc_init();
+
+class Global {
+public:
+    Global(int i)
+        : m_i(i)
+    {
+        __malloc_init();
+        __stdio_init();
+    }
+
+    int get_i() const { return m_i; }
+
+private:
+    int m_i = 0;
+};
+
+// This object exists to call __stdio_init and __malloc_init. Also to show that global vars work
+Global g_glob { 5 };
+
+extern "C" {
+int global_lib_variable = 1234;
+
+void global_lib_function()
+{
+    printf("Hello from Dynamic Lib! g_glob::m_i == %d\n", g_glob.get_i());
+}
+
+const char* other_lib_function(int my_argument)
+{
+    dbgprintf("Hello from Dynamic Lib, now from the debug port! g_glob::m_i == %d\n", g_glob.get_i());
+
+    int sum = my_argument + global_lib_variable;
+
+    // FIXME: We can't just return AK::String::format across the lib boundary here.
+    //     It will use malloc from our DSO's copy of LibC, and then probably be free'd into
+    //     the malloc of the main program which would be what they call 'very crash'.
+    //     Feels very Windows :)
+    static String s_string;
+    s_string = String::format("Here's your string! Sum of argument and global_lib_variable: %d", sum);
+    return s_string.characters();
+}
+}