Add Glibc and Bionic predefined global versions

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.
Glibs might be the better identifier, but why nuke it for other platforms? Isn't this the usual runtime for all non-Windows platforms?