It turns out that in x86-64 mode, address and operand size overrides are
ignored by the processor. (Tested that on a real machine.)
Even libopcodes gets this wrong...
It is possible to configure the build process such that decoding of 32
bit and 64 bit instructions can be chosen at runtime using an additional
parameter of the decode function. The header file is now entirely
architecture-independent and no longer required any previous defines.
Decoding x86-64 still requires a 64-bit pointer size.
This allows to decode x86-32 machine code on a 64-bit platform (but
not vice versa). As a side-effect, we also get rid of pointer-size
detection for architecture selection.
