dynlink.c: pass gnu-hash table pointer to gnu_lookup

dynlink.c: pass gnu-hash table pointer to gnu_lookup

The callers need to check the value of the pointer anyway, so make
them pass the pointer to gnu_lookup instead of reloading it there.

Reorder gnu_lookup arguments so that always-used ones are listed
first. GCC can choose a calling convention with arguments in registers
(e.g. up to 3 arguments in eax, ecx, edx on x86), but cannot reorder
the arguments for static functions.