mini-os: Make x86_64 support work again
mini-os: Fixed "make clean"

Was failing to delete object files from some subdirectories.
mini-os: Removed test_xenbus from idle thread

Looks like this was just left-over debug code.
mini-os: Fixed #ifdef test
mini-os: Made off_t type signed

POSIX requires this.
mini-os: Make .o files depend on includes

Before, only the final link step depended on setting up the includes
directory, making the build unreliable.
mini-os: When panicing, show which exception vector was called