[runtime] First pass of cross compilation cleanup. Allow offsets and alignment to be externally calculated.
Revert "Remove duplicate System.Web in mcs/class/Makefile."

This reverts commit 5fbb351375a38232ff9cb5ca6602f183321e0a47.

This breaks the build because System.Runtime.Remoting needs System.Web.
mcs/class/Makefile: Merge common_dirs into net_2_0_dirs.

They're used in the same places so we may as well simplify.
mcs/class/Makefile: Remove duplicate System.Web entry.

Second take on 5fbb351375a38232ff9cb5ca6602f183321e0a47.