Try to guess a reasonble default for Tool RID

This helps with cases where folks invoke msbuild directly without going
through build.sh or build.cmd.  On Windows it's very easy to guess but
on "Unix" it's harder so right now we assume that the Target OS and Host
OS are the same (of course, this could be overridden).

build.sh

@@ -129,7 +129,7 @@ build_managed_corefx()
     __buildproj=$__scriptpath/build.proj
     __buildlog=$__scriptpath/msbuild.log
 
-    MONO29679=1 ReferenceAssemblyRoot=$__referenceassemblyroot mono $__msbuildpath "$__buildproj" /nologo /verbosity:minimal "/fileloggerparameters:Verbosity=normal;LogFile=$__buildlog" /t:Build /p:OSGroup=$__BuildOS /p:UseRoslynCompiler=true /p:COMPUTERNAME=$(hostname) /p:USERNAME=$(id -un) /p:TestNugetRuntimeId=$__TestNugetRuntimeId "$@"
+    MONO29679=1 ReferenceAssemblyRoot=$__referenceassemblyroot mono $__msbuildpath "$__buildproj" /nologo /verbosity:minimal "/fileloggerparameters:Verbosity=normal;LogFile=$__buildlog" /t:Build /p:OSGroup=$__BuildOS /p:UseRoslynCompiler=true /p:COMPUTERNAME=$(hostname) /p:USERNAME=$(id -un) /p:TestNugetRuntimeId=$__TestNugetRuntimeId /p:ToolNugetRuntimeId=$__TestNugetRuntimeId "$@"
     BUILDERRORLEVEL=$?
 
     echo

dir.props

@@ -188,6 +188,17 @@
     <TestNugetRuntimeId Condition="'$(OSGroup)'=='FreeBSD'">ubuntu.14.04-x64</TestNugetRuntimeId>
   </PropertyGroup>
 
+  <!-- Provide defaults for ToolNugetRuntimeId -->
+  <PropertyGroup Condition="'$(ToolNugetRuntimeId)'== ''">
+    <ToolNugetRuntimeId Condition="'$(OsEnvironment)'=='Windows_NT'">win7-x64</ToolNugetRuntimeId>
+
+    <!-- This is a bit of a hack because inside MSBuild we have no real concept of the host distro, so we'll assume
+         that if you are building on not windows, your host OS is the same as your target.  If that's not
+         the case, you'll have to provide your own ToolNugetRuntimeId, which is not the end of the world (build.sh will
+         do this for you, for example). -->
+    <ToolNugetRuntimeId Condition="'$(OsEnvironment)'=='Unix'">$(TestNugetRuntimeId)</ToolNugetRuntimeId>
+  </PropertyGroup>
+
   <!-- Set up Default symbol and optimization for Configuration -->
   <PropertyGroup Condition="'$(ConfigurationGroup)' == 'Debug'">
     <DebugSymbols Condition="'$(DebugSymbols)' == ''">true</DebugSymbols>