Keep top page on CloudABI to work around AMD Ryzen stability issues.

Similar to r321899, reduce sv_maxuser by one page inside of CloudABI.
This ensures that the stack, the vDSO and any allocations cannot touch
the top page of user virtual memory.

Considering that CloudABI userspace is completely oblivious to virtual
memory layout, don't bother making this conditional based on the CPU of
the running system.

Reviewed by: kib, truckman
Differential Revision: https://reviews.freebsd.org/D11808