libphobos: Merge common version blocks for core.sys.posix.sys.msg.

This is a continuation of simplifying C bindings so there aren't dozens
of duplicated code for each architecture.  For this particular module,
it now more closely resembles how glibc arranges msq.h, fixing a couple
of targets in the process, notably X32.

Backport from upstream druntime 2.084.

Reviewed-on: https://github.com/dlang/druntime/pull/2362

From-SVN: r267192