Issue 8370 (comment 2).  Fix deprecated message building phobos.

Issue 8370 (comment 2).  Fix deprecated message building Phobos regarding MAP_ANONYMOUS
This line is in `version(Posix)`, so this change would be wrong in non-linux posix platforms.

And, `std.mmfile` does not import `core.sys.linux.sys.mman`. So I think a better change is:

```
    version(linux) import core.sys.linux.sys.mman : MAP_ANON;
    // override core.sys.posix.sys.mman.MAP_ANON in inner scope
    flags |= MAP_ANON;
```