This sample can be used to create a reverse shell when combined with the
shell64 sample:

    cat examples/{tcp64,shell64}.asm >code.asm
    make
    ...

Signed-off-by: Malfurious <m@lfurio.us>

The shell-spawning shellcodes are rewritten to address the following
concerns:

    - The array parameters to execve are now set properly, to valid
      arrays on the stack, instead of NULL pointers.

    - The cdq instruction is no longer used to sign-extend the rax
      register, since it has not been producing the expected results in
      gdb.

    - Labels, sections, and other file metadata are removed in order to
      support concatenation of shellcode samples to make more complex
      code.

Signed-off-by: Malfurious <m@lfurio.us>

This is mainly done to keep the top working directory (where the
Makefile lives) cleaner.

Signed-off-by: Malfurious <m@lfurio.us>