We need to pass the location of the icu build root for the BUILD platform with
the --with-cross-build option when configuring icu for cross-compilation (to
the HOST platform). I.e. we always need to build it for the BUILD
platform. Surely this could be done in a more elegant way, but I got lost in
the mess, and just hacked it in the cross-toolset rule in Makefile.in.

Change-Id: Ia5e9d13d01067c6dded1fdc86c56f8f39aae6bc8