[v2] preloader: Allow a debugger to find ld.so's rendezvous-struct

[Keno Fischer]

Whether .dynamic is writable depends on the architecture (I believe
the only one on which it isn't is MIPS, but I could be wrong about
that). Looks like binutils doesn't take that into account when issuing
this diagnostic in the assembler and expects .dynamic to have
SHF_ALLOC only. Probably something to complain about to them. I don't
believe there's an actual problem though, just an incorrect
diagnostic.