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/process.cpp

@@ -1,11 +1,13 @@
 #include "process.h"
+#include "errno.h"
 #include <Kernel/Syscall.h>
 
 extern "C" {
 
 int spawn(const char* path)
 {
-    return Syscall::invoke(Syscall::Spawn, (dword)path);
+    int rc = Syscall::invoke(Syscall::Spawn, (dword)path);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
 }