tunables: Make tunable_list relro

tunables: Make tunable_list relro

As of now we don't need tunables to be writable after they have been
set initially, so there is no point in having them writable.  Put
tunable_list in .data.rel.ro so that it is set as read-only after
relocation.  This also allows us to move some of the dl_* variables
that are tunables controlled into the tunables infrastructure instead
of having two copies.

In future if we ever need specific tunables to be writable at runtime,
we can split the tunable_list into two.

Regression tested on x86_64 to verify that tests continue to pass.

	* scripts/gen-tunables.awk: Add attribute_relro to
	tunable_list.