lang/smalltalk: use modern memory management

The memory allocator defaults to some horrifying complex code to manage
mmap'd allocations. On system where MAP_NORESERVE is defined (Linux) it
uses a much simpler approach relying on memory overcommit. Enable this
code on FreeBSD by defining MAP_NORESERVE to 0 (all allocations
are MAP_NORESERVE on FreeBSD unless a sysctl is set).

Entierly disable the other code path as it (somewhat gratutiously) uses
sbrk.

Approved by: danfe (maintainer)
Sponsord by: DARPA
Differential Revision: https://reviews.freebsd.org/D42974