Provide build support for x86_64

[bitigchi]

See title.

[thaflo]

There is missing a "please" in the title, so I'm adding that.
On 64bit, it doesn't build but that would be fine! Please, stippi :-)

[stippi]

I'm afraid I am quite out of touch with the code-base. No idea when I will have time to look into it. It's not about "build" support only, I would have to port the code-base to 64bit support. A lot of it will compile regardless, but some other parts will not. I will gladly accept any help.

[diversys]

Added 64bit recipe in haikuports/haikuports@5404d0b.
Compiles and runs seemingly fine.
32bit build is broken though https://build.haiku-os.org/buildmaster/master/x86_gcc2/logviewer.html?buildruns/2161/builds/77144.log