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