Add PEXT software implementation

For development/debug purposes.

No functional change.
Implement PEXT based attacks

According to:

https://chessprogramming.wikispaces.com/BMI2#PEXTBitboards

No functional change.
Switch to hardware PEXT

Retire software pext and introduce hardware
call when USE_PEXT is defined during compilation.

This is a full complete implementation of sliding
attacks using PEXT.

No functional change.
Add ARCH x86-64-bmi2 support

Intel Haswell and newer CPUs can calculate sliders
attacks using special PEXT asm instructions instead
of magic bitboards. This gives a +3% speed up.

To enable it just compile with ARCH=x86-64-bmi2

No functional change.
Retire signature-build

Does not seem used.

No functional change.