While at the Dark Side of the Moon screen, press Select, L, Right, B, L, R, Left.  Then go to the Build Secret Base option, press A, and type in GOD.

Unlimited Ammo

While at the Dark Side of the Moon screen, press Select, L, Right, B, L, R, Left.  Then go to the Build Secret Base option, press A, and type in AMMO.

Unlock all Weapons

While at the Dark Side of the Moon screen, press Select, L, Right, B, L, R, Left.  Then go to the Build Secret Base option, press A, and type in WEAP.