WIP: Separate the linux kernel, glibc, and bionic APIs where appropriate

Building on submitted dmd pulls [#4111](https://github.com/D-Programming-Language/dmd/pull/4111) and [#3643](https://github.com/D-Programming-Language/dmd/pull/3643) and druntime pull [#1010](https://github.com/D-Programming-Language/druntime/pull/1010), this pull substitutes the new `CRuntime_Glibc` and `CRuntime_Bionic` predefined global versions where appropriate.  After those dmd pulls are merged, `linux` will be defined for both desktop/server linux and Android, as opposed to the current situation where either `linux` or `Android` alone is expected to be defined (which is why I had to change the order in `std.system`).  dmd #4111 will have to be merged before this pull will compile and pass tests.