Add predefined global version for Glibc

This is necessary to start separating out linux and glibc APIs in druntime.  Right now, `version(linux)` means linux+glibc APIs in druntime/phobos, whereas afterwards `linux` will refer to linux kernel APIs, `CRuntime_Glibc` to glibc APIs, and `CRuntime_Bionic` to bionic APIs, at least in druntime and phobos.  I don't care what names are actually used, this is just what seemed to make sense.  I've updated [my pull which added Android support to dmd, so you can see how they'll be defined](https://github.com/D-Programming-Language/dmd/pull/3643/files#diff-791916028fd5cf71b2903bad5fe4ddf3L618).  I've started updating druntime to use these new versions, the changes should be completely transparent to users.

I'm not sure why the C runtimes were left out of the list of predefined global versions in cond.c, so I added them.  Also, I noticed that `D_InlineAsm` isn't in the list either, [presumably because it predated 64-bit support and is only around for legacy reasons](https://github.com/D-Programming-Language/dmd/commit/788b8a6285a46f1a1ce2952d42e9bac3c9dbe7f7#diff-791916028fd5cf71b2903bad5fe4ddf3L287), so I added a comment to that effect.  Let me know if it should be added to the list in cond.c anyway.