[builds] Download the netcore 3.1 assemblies to use as reference assemblies for .NET 5.

I couldn't figure out how to make nuget install these into the system, so I
decided to just download the package locally instead.

This is just temporary until we get real .NET 5 reference assemblies.

Make.config

@@ -374,6 +374,9 @@ endif
 
 DOTNET_VERSION=3.1.100
 DOTNET_URL=https://download.visualstudio.microsoft.com/download/pr/787e81f1-f0da-4e3b-a989-8a199132ed8c/61a8dba81fbf2b3d533562d7b96443ec/dotnet-sdk-3.1.100-osx-x64.pkg
+DOTNET_BCL_REF_URL=https://www.nuget.org/api/v2/package/Microsoft.NETCore.App.Ref/3.1.0
+DOTNET_BCL_REF_NAME=microsoft.netcore.app.ref.3.1.0.nupkg
+DOTNET_BCL_DIR:=$(abspath $(TOP)/builds/downloads/$(basename $(DOTNET_BCL_REF_NAME)))/ref/netcoreapp3.1
 	
 .SUFFIXES:
 MAKEFLAGS += --no-builtin-rules

builds/Makefile

@@ -15,14 +15,20 @@ download-mono: downloads/$(basename $(MONO_IOS_FILENAME)) downloads/$(basename $
 
 downloads/$(basename $(MONO_IOS_FILENAME)): MONO_URL=$(MONO_IOS_URL)
 downloads/$(basename $(MONO_MAC_FILENAME)): MONO_URL=$(MONO_MAC_URL)
+downloads/$(DOTNET_BCL_REF_NAME): MONO_URL=$(DOTNET_BCL_REF_URL)
 
 include $(TOP)/mk/colors.mk
 
+DOWNLOADS = \
+	downloads/$(MONO_IOS_FILENAME) \
+	downloads/$(MONO_MAC_FILENAME) \
+	downloads/$(DOTNET_BCL_REF_NAME) \
+
 # This target downloads the mono archives, there's one for Xamarin.iOS and one for Xamarin.Mac.
 # If doing many clean builds, it's possible to copy the downloaded zip file to ~/Library/Caches/xamarin-macios
 # to avoid having to download it every time. The zip files have to be copied manually, otherwise
 # we'd end up filling up a lot of hard drives around the world.
-downloads/$(MONO_IOS_FILENAME) downloads/$(MONO_MAC_FILENAME):
+$(DOWNLOADS):
 	$(Q) mkdir -p downloads
 	$(Q) echo "Downloading $(MONO_URL)..."
 	$(Q) if test -f ~/Library/Caches/xamarin-macios/$(notdir $@); then \
@@ -49,9 +55,25 @@ downloads/%: downloads/%.7z
 	$(Q) mv $@.tmp $@
 	$(Q) echo "Unzipped $*."
 
+downloads/%: downloads/%.nupkg
+	$(Q) echo "Unzipping $*..."
+	$(Q) rm -Rf $@.tmp
+	$(Q) unzip -d $@.tmp $<
+	$(Q) find $@.tmp -exec touch {} +
+	$(Q) mv $@.tmp $@
+	$(Q) echo "Unzipped $*."
+
 .stamp-download-mono: download-mono $(TOP)/Make.config $(TOP)/mk/mono.mk
 	$(Q) touch $@
 
+DOTNET_DOWNLOADS = \
+	downloads/$(basename $(DOTNET_BCL_REF_NAME)) \
+
+dotnet: $(DOTNET_DOWNLOADS)
+ifdef ENABLE_DOTNET
+all-local:: $(DOTNET_DOWNLOADS)
+endif
+
 clean-local::
 	$(Q) rm -Rf downloads .stamp-download-mono