Currently when we get target consts with target.ConstMap["name"]
during target initialization, we just get 0 for missing consts.
This is error-prone as we can mis-type a const, or a const may
be undefined only on some archs (as we have common unix code
shared between several OSes).
Check that all the consts are actually defined.
The check detects several violations, to fix them:
1. move mremap to linux as it's only defined on linux
2. move S_IFMT to openbsd, as it's only defined and used on openbsd
3. define missing MAP_ANONYMOUS for freebsd and netbsd
4. fix extract for netbsd
This is mostly copied form linux.
We probably need better support for sharing descriptions
between multiple OSes. But there are lots of differences,
so this is not trivial.