If the kernel doesn't have MADV_MERGEABLE, stop asking for it.

Note that a dynamically-linked binary will still probably see two attempts ---
one by the dynamic linker (which will set its copy of the flag so it won't try
again) and then one by the executable itself (which gets a new uninitialized
copy of the flag).

Change-Id: Id6b7e47780f0f24d2ca0384a75373f4824fa8f12