[netcore] Use mono-netcore to run real world apps using `dotnet run` (mono/mono#15942)

So currently we have two .NET Core:

1) `mono-repo/.dotnet` - we use it to bootstrap and build stuff (the SDK version is specified in `global.json`) - if you open `netcore/Makefile` it's referred there as `$(DOTNET)` variable.
2) `mono-repo/netcore/.dotnet` - it's just a runtime + CoreFX libs (it doesn't contain SDK stuff, templates, msbuild, etc) - we use it only to run tests because this runtime is synchronized with corefx tests in `eng/Versions.prop` file which is updated by a bot and `netcore/shared` sources for our System.Private.CoreLib also depend on that exact runtime version (the sources also updated by the bot).

This PR introduces a rule to copy mono bits to the `mono-repo/.dotnet` folder to be able to run real world apps (at your own risk) using mono-netcore (so when you run `mono-root/.dotnet/.dotnet run -c Release` for an app - even MSBuild will use mono runtime to build that app). However in theory we need exact SDK for the runtime version we have but I have no idea where and how to get one.

BTW, currently MSBuild crashes when it's powered by mono on `$(DOTNET) build` command for a hello world.
```
monoeg_assertion_message
mono_domain_assembly_preload
invoke_assembly_preload_hook
...
at <unknown> <0xffffffff>
at System.Reflection.RuntimeAssembly:GetExportedTypes <0x000a2>
at AssemblyInfoToLoadedTypes:ScanAssemblyForPublicTypes <0x0008e>
```

Commit migrated from https://github.com/mono/mono/commit/d0aa0310ee386433a9ef0b4c15ac07c928211bcb

diff --git a/src/mono/netcore/Makefile b/src/mono/netcore/Makefile
index 902c0c4..6cc2710 100644 (file)
--- a/src/mono/netcore/Makefile
+++ b/src/mono/netcore/Makefile
@@ -9,6 +9,10 @@ ROSLYN_VERSION:=  $(shell cat ../eng/Versions.props | sed -n 's/.*MicrosoftNetCo
 # Extracted from https://dotnetcli.blob.core.windows.net/dotnet/aspnetcore/Runtime/master/latest.version
 ASPNETCOREAPP_VERSION := 3.0.0-preview-18614-0151
 
+# runtime version used by $(DOTNET) - local .net core sdk to bootstrap stuff
+# it doesn't match NETCOREAPP_VERSION
+BOOTSTRAP_RUNTIME = $(shell ls ../.dotnet/shared/Microsoft.NETCore.App)
+
 ifeq ($(HOST_PLATFORM),win32)
 PLATFORM_AOT_SUFFIX := .dll
 PLATFORM_AOT_PREFIX :=
@@ -95,6 +99,12 @@ run-aspnet-sample: prepare
        cp System.Private.CoreLib/bin/$(COREARCH)/System.Private.CoreLib.dll sample/AspNetCore/bin/Debug/netcoreapp3.0/$(RID)/publish/
        COMPlus_DebugWriteToStdErr=1 ./dotnet --fx-version "$(ASPNETCOREAPP_VERSION)" sample/AspNetCore/bin/Debug/netcoreapp3.0/$(RID)/publish/AspNetCore.dll
 
+# makes $(DOTNET) to use mono runtime (to run real-world apps using '$(DOTNET) run')
+patch-local-dotnet: prepare
+       cp ../mono/mini/.libs/libmonosgen-2.0$(PLATFORM_AOT_SUFFIX) ../.dotnet/shared/Microsoft.NETCore.App/$(BOOTSTRAP_RUNTIME)/$(PLATFORM_AOT_PREFIX)coreclr$(PLATFORM_AOT_SUFFIX)
+       cp System.Private.CoreLib/bin/$(COREARCH)/System.Private.CoreLib.dll ../.dotnet/shared/Microsoft.NETCore.App/$(BOOTSTRAP_RUNTIME)
+       cp System.Private.CoreLib/bin/$(COREARCH)/System.Private.CoreLib.pdb ../.dotnet/shared/Microsoft.NETCore.App/$(BOOTSTRAP_RUNTIME)
+
 # COREHOST_TRACE=1 
 SHAREDRUNTIME := shared/Microsoft.NETCore.App/$(NETCOREAPP_VERSION)