32-bit mips SMP is unsupported

Per discussions on mips@, 32-bit mips SMP is now unsupported. The
files in the tree will compile for a while longer, but when the
atomic_swap_64 or similar atomic enters into the MI part of the tree,
as currently foreseen sometime next year, these ports will start to no
longer link. The JZ4780 is the only such system we have.

The UP version of this chip is unaffected by this, and will remain
supported.

Discussed on: mips@
Relnotes: yes