Rejigger OSMem.my_mmap to allow building on Mac

Authored by goldfire.

Description

Rejigger OSMem.my_mmap to allow building on Mac

Previously, the prot and flags variables were set but never used
on Mac (darwin). This caused a warning, and the build setup stopped
compilation. This commit is intended simply to omit these variables
when building with darwin_HOST_OS set. No change in behavior on any
platform is intended.