Maybe for some stuff while building cross compiler? What would be really strange, native build using cross compiler needs no libm but cross compiler building on i686 does ;-)
I think the problem was only with the C++ libraries.
Mystery solved. It's true I removed all traces of libm.a but I didn't remove the header file (math.h) and this is it -- libstdc++ needs math.h for declaration of ::acos, ::asin etc. I just tested it's totally OK just include math.h (without any other files) into include dir for my cross environment and whole toolchain is compiled OK (native and cross).
So maybe you could Vincent just make some skeleton for math.h and include this into your gcc patch -- one library down for compilation (even when it compiles quite quickly).