gecko: declare NOT_FOR_ARCHS=i386
The building and linking of the rust parts on x86 needs a larger
address space than a 32bit platform can provide (at most 4GB).
The gecko ports have been failing for quite a while on the official
builders with an "rustc-LLVM ERROR: out of memory" message, e.g.
https://pkg-status.freebsd.org/beefy18/data/144i386-default/6f1fe399c86f/logs/thunderbird-153.0.2.log
We can as well stop trying to save the CPU cycles leading up to
this error.
(cherry picked from commit 368d875486b966c782bc2a7d91f31f1d6ef44175)