* [dotnet] Ship libxamarin.dylib and friends.
Add libxamarin[-debug].[a|dylib] to the NuGets.
* [dotnet] Create a DOTNET_PLATFORMS variable in Make.config.
Create a DOTNET_PLATFORMS variable in Make.config and use it everywhere
instead of the PLATFORMS variable we were defining in multiple Makefiles.
Also move the creation of the DOTNET_<platform>_RUNTIME_IDENTIFIERS variables
from dotnet/Makefile to Make.config, it'll soon be needed elsewhere as well.
* [runtime] Conditionally include bits.
* Make the contents of the DOTNET_[PLATFORMS|RUNTIME_IDENTIFIERS] variables depend on the INCLUDE_[IOS|TVOS|WATCH] variables.
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