Add preserve-libs for stable branch (not default)

Add preserve-libs for stable branch (not default)

EAPI 5 supports automatic rebuilds via the slot-operator and sub-slots,
which makes preserve-libs much more user-friendly, since it makes
@preserved-rebuild unnecessary (also see bug #364425 for explanation of
@preserved-rebuild shortcomings). Therefore, enable preserve-libs for
the stable branch, but not by default. After EAPI 5 is widely adopted,
we can consider enabling preserve-libs by default.