Implement errno in LibC.

This also meant I had to implement BSS (SHT_NOBITS) sections in ELFLoader.
I also added an strerror() so we can print out what the errors are.

LibC/entry.cpp

@@ -2,8 +2,12 @@
 
 extern "C" int main(int, char**);
 
+int errno;
+
 extern "C" int _start()
 {
+    errno = 0;
+
     // FIXME: Pass appropriate argc/argv.
     int status = main(0, nullptr);