[mcs] Add platform specific files (#4837)

[mcs] Add platform specific files (#4837)

* [mcs] Rework %-recursive target to explicitely list subdirectories

* [mcs] Rename PLATFORM to BUILD_PLATFORM

* [mcs] Add platform specific files

This is to support adding corefx platform specific files. To do that, we now support `mcs/class/corlib/linux_net_4_x_corlib.dll.sources` for example, on top of `mcs/class/corlib/net_4_x_corlib.dll.sources` and `mcs/class/corlib/corlib.dll.sources`.

The 3 platforms that are supported are: `linux`, `darwin` and `win32`.

* [mcs] Build cross-platform bcl in separate folder