Fix build and tests with --disable-tunables

Remove unused code and declare __libc_mallopt when !IS_IN (libc) to
allow the debug hook to build with --disable-tunables.

Also, run tst-ifunc-isa-2* tests only when tunables are enabled since
the result depends on it.

Tested on x86_64.

Reported-by: Matheus Castanho <msc@linux.ibm.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>