[bcl] Make monolite platform specific (#4910)

[bcl] Make monolite platform specific (#4910)

mscorlib.dll etc will be platform specific in the future. This also
means that we need to adapt monolite since it'll become platform
specific once we start the divergence.

monolite is essentially a repackaging of the "build" profile which
means we need to turn that profile into multi-platform profile
like net_4_x. We'll still have a symlink from build -> build-darwin
and the monolite which is downloaded or packaged in the tarball
is expanded into mcs/lib/monolite-<platform> depending on the platform
you're building on.

I had to move the get-monolite-latest target from the top-level
Makefile.am into mcs/class/Makefile so it has access to HOST_PLATFORM,
but it arguably should've been there anyway before.