Use 64-bit code

Resolves #1 (closed).

I tried to make only the minimum required changes to port the programs to 64-bit. The rewritten code is working as expected.

Where C and asm interoperate, I needed to explicitly disable position-independent executables. Most distros enable PIE on x86_64 by default. See Stack Overflow.

Note that chapter2/pi/v2 calculates a wrong result when MAX_THREADS is greater than one. I assume, this is intended as the v3 introduces mutexes.

Running $ make in the top level directory of the repository still fails, since there is chapter2/fpu beside chapter2/fpu64. Therefore I would suggest to remove fpu. If you are okay with it, I would add another commit.