try multiproc build (#3068)

build.cmd

@@ -527,8 +527,8 @@ if defined TF_BUILD (
     git fetch --all
 )
 
-REM set msbuildflags=/maxcpucount %_nrswitch% /nologo
+set msbuildflags=/maxcpucount %_nrswitch% /nologo
-set msbuildflags=%_nrswitch% /nologo
+REM set msbuildflags=%_nrswitch% /nologo
 set _ngenexe="%SystemRoot%\Microsoft.NET\Framework\v4.0.30319\ngen.exe"
 if not exist %_ngenexe% echo Error: Could not find ngen.exe. && goto :failure