Kernel: Demangle userspace ELF symbols in backtraces

Turns out we can use abi::__cxa_demangle() for this, and all we need to
provide is sprintf(), realloc() and free(), so this patch exposes them.

We now have fully demangled C++ backtraces :^)

Kernel/StdLib.cpp

@@ -174,4 +174,13 @@ char* strstr(const char* haystack, const char* needle)
     ASSERT_NOT_REACHED();
 }
 
+void* realloc(void* p, size_t s)
+{
+    return krealloc(p, s);
+}
+
+void free(void* p)
+{
+    return kfree(p);
+}
 }