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 @@
     <!-- No need to track public APIs of this tool. -->
     <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>