Pass 0 instead of uvm_map_hint() to uvm_map() in exec_md_map() to figure out

Pass 0 instead of uvm_map_hint() to uvm_map() in exec_md_map() to figure out
where to put the fpu assist page, for uvm_map_hint() may return an address
outside userland bounds due to aggressive randomization. Passing zero will
still get a random address, but correctly bounded.