Untangle ifdef forest in sys_mmap64. No code changes

Untangle ifdef forest in sys_mmap64. No code changes

After careful analysis, it looks like !LINUX and ALPHA
pass all seven parameters in registers; and in all other cases
parameters are on stack (pointed to by tcp->u_arg[0]).
In light of this, reorganize ifdefs, making them simpler,
without changing any logic.
After this, it's apparent we use tcp->u_arg[4,5,6] and possibly
[7] without checking that it's valid to do so.
So far, just add a comment about this.

* mem.c (sys_mmap64): Rewrite ifdefs in a much simpler way.
Add comments about apparent bugs.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>