Provide a 64 bits version of Pliant

Pliant provides a built in code generator for i386 processors.
Some modifications should be applied in the code generator to use 64 bits instructions set extension introduced by AMD when _64BITS_ is defined.

Then, the biggest task will be to update various parts of the code interacting with the kernel and external libraries, because Pliant assumes that on a 64 bits plateform, an 'Int' size is 64 bits, whereas C assumes that it's 32 bits.