Don't use an apphost for RazorSyntaxGenerator

The RazorSyntaxGenerator tool is excluded from source-build, but the build

system tries to find the apphost matching the TFM anyway. This project targets

netcoreapp3.1, so the build fails on s390x because the no apphost exists for

that on s390x.

--- a/src/razor-compiler/src/tools/RazorSyntaxGenerator/RazorSyntaxGenerator.csproj

+++ a/src/razor-compiler/src/tools/RazorSyntaxGenerator/RazorSyntaxGenerator.csproj

@@ -10,6 +10,7 @@

     <AddPublicApiAnalyzers>false</AddPublicApiAnalyzers>

     <ExcludeFromSourceBuild>true</ExcludeFromSourceBuild>

+    <UseAppHost>false</UseAppHost>

   </PropertyGroup>

 </Project>

--- a/src/razor-compiler/src/perf/Microbenchmarks/Microsoft.AspNetCore.Razor.Microbenchmarks.csproj

+++ a/src/razor-compiler/src/perf/Microbenchmarks/Microsoft.AspNetCore.Razor.Microbenchmarks.csproj

@@ -7,6 +7,7 @@

     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>

     <ExcludeFromSourceBuild>true</ExcludeFromSourceBuild>

     <IsPackable>false</IsPackable>

+    <UseAppHost>false</UseAppHost>

   </PropertyGroup>

   <ItemGroup>