utils: remove Host, Build, Target duplication

Adopt the `$Arch` parameter uniformly. If something is built for the
host we can use `$HostArch` explicitly (which identifies the
environment that the toolchain will run on). `$BuildArch` gives us an
explicit architecture when building for the machine where the toolchain
is being built. For the target libraries, we simply pass in the
appropriate `$Arch` value.

Author: compnerd

utils/build-windows-toolchain.bat

@@ -73,7 +73,7 @@ call :CloneRepositories || (exit /b 1)
 :: We only have write access to BuildRoot, so use that as the image root.
 powershell.exe -ExecutionPolicy RemoteSigned -File %~dp0build.ps1 ^
   -SourceCache %SourceRoot% ^
-  -BinaryCache %BuildRoot% ^
+  -BinaryCache "%BuildRoot%\" ^
   -ImageRoot %BuildRoot% ^
   %SkipPackagingArg% ^
   %TestArg% ^