lang/mono5.10,lang/mono: Attempt to fix build on aarch64

The Roslyn C# compiler has a concurrency problem on aarch64:
https://github.com/mono/mono/issues/7017 (not FreeBSD specific)
so the workaround is to disable parallelism… so the .NET libraries are built
very very slowly

PR: 229710
Approved by: portmgr (build fix blanket)