Filter out mmap tasks for non-x86 build variants

We currently run integration tasks with mmap as the storage engine on some platform architectures that don't support it:

MMAPv1 is supported on 64-bit Intel architectures (x86_64) only. MongoDB returns an error when starting a mongod using MMAPv1 on an unsupported architecture.

(from: https://docs.mongodb.com/v4.0/core/mmapv1/)

We should make sure that we filter out these unsupported integration tasks (i.e. for an unsupported architecture that runs all "3.6" tasks, we should replace that with ".3.6 !.mmap"). This applies mostly to the PPC variants, though there may be others.