On Linux, Simply Fortran uses the system's Fortran compiler.  If you are on a 32-bit Linux distribution, it will produce 32-bit code, and likewise, for a 64-bit distribution, it will produce 64-bit code.  You could theoretically point Simply Fortran at a cross compiler on Linux so that you could produce 32-bit code on a 64-bit distribution if you wanted to.

Since we do not control the compiler with Simply Fortran for GNU/Linux, we've disabled the "Architecture" selector on Linux.  Most Linux Fortran compilers are not going to be "multilib," so it wouldn't make sense to imply that switching the architecture would be possible.  That selector won't be added to Simply Fortran for GNU/Linux.