Disregard the previous mail, I figured most of it out.

The attached patch does the following:
   1.) Adds the name of the dynamic linker to gnu/packages/bootstrap.scm
   2.) Adds code to gnu/package/gcc to substitute the default dynamic 
linker in gcc/config/arch/gnu.h for the one to be used.
   3.) Changes how the build system arguments for cross-gcc are created:
     A.) They're now in a separate procedure, reflecting how it's done 
in vanilla guix.
     B.) A bug is fixed that caused the native ld.so to be picked over 
the target one.
     C.) Fixed the 'set-cross-path phase so that it also checks for 
hurd-related packages, and checks package existence before passing 
anything to the string-handling functions.

The current issue is that, when building cross-gcc, when it reaches 
libgomp, configure fails when trying to create a dummy binary. For some 
reason, the compiled cross-compiler needs --rpath to specify where 
shared libraries are, as without it, it can't find libmachuser.so and 
libhurduser.so, which causes it to fail.

-- 
Marek.