Allow building of 32-bit Linux version of SRB2 2.0 (as the 64-bit Linux version is extremely unstable)

[jroyvde]

The 64-bit Linux version of SRB2 2.0 has an issue where it crashes intermittently during gameplay. Sometimes after a few minutes, sometimes just a few seconds after starting gameplay - but in my testing it always happens sooner or later. This isn't an issue with how srb2bld builds it, the issue is also present in the official build distributed on the SRB2 website here:
http://alam.srb2.org/SRB2/2.0.7-Final/Linux64/amd64/Release/lsdlsrb2

The 32-bit version, however, works just fine. You can download and test it here:
https://alam.srb2.org/SRB2/2.0.7-Final/Linux/Release/lsdlsrb2

Because of this issue, I think that the 32-bit version of 2.0 should be used instead of the 64-bit version for Linux. If not by default, then it a parameter or prompt to choose the 32-bit version would also be fine.

[Bijman]

I got SIGSEGV errors when player's character rolls on 64-bit Linux version and it does happen less in 32-bit version in binary files distributed in SRB2 website and compiled with srb2bld, so it seems you are right.