64-bit linux (x86_64) support

These changes add support for compiling myppy as 64bit on linux.

It is now possible on 64bit linux to specify if myppy should be compiled as 32 or 64bit. To do that, the user can tell the architecture explicitly

#> myppy PATH/TO/ENV init  [ 32bit | 64bit ]

By default myppy will compile on 32bit linux as 32bit on 64bit linux as 64bit.