authexec: lion & older only

See formula for explanation.

Closes #34039.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>
ld64: transitioning requirements

Follows up on Misty’s comments in #34039. Hopefully this change looks
okay to Misty.

Closes #34041.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>