Avoid ngen for `amd64\` folder DLLs

[rainersigwald]

We should not load assemblies from the bin\amd64 subfolder of MSBuild in normal operation--VS, MSBuild.exe, and MSBuildLocator should load the single copies from bin\ instead.

Some tools load MSBuild assemblies directly from bin\amd64 and as a result we've had to ship should-be-redundant copies of assemblies there in several cases. We should avoid this where possible.

Today, most of those assemblies are marked for ngen, and further with the default ngen that uses vsn.exe.config (the in-VS binding redirects) AND FURTHER for all architectures:

msbuild/src/Package/MSBuild.VSSetup/files.swr

Lines 180 to 190 in a71903d

	folder InstallDir:\MSBuild\Current\Bin\amd64
	file source=$(X64BinPath)MSBuild.exe vs.file.ngenArchitecture=x64
	file source=$(TaskHostX64BinPath)MSBuildTaskHost.exe
	file source=$(X64BinPath)MSBuild.exe.config
	file source=$(TaskHostX64BinPath)MSBuildTaskHost.exe.config
	file source=$(X86BinPath)Microsoft.Build.dll vs.file.ngenArchitecture=all
	file source=$(X86BinPath)Microsoft.Build.Framework.dll vs.file.ngenArchitecture=all
	file source=$(FrameworkBinPath)x64\Microsoft.Build.Framework.tlb
	file source=$(X86BinPath)Microsoft.Build.Tasks.Core.dll vs.file.ngenArchitecture=all
	file source=$(X86BinPath)Microsoft.Build.Utilities.Core.dll vs.file.ngenArchitecture=all