Basic multilib settings need a sanity check to ensure they are both reasonable and defined properly.

For instance, adding a typo when setting the DEFAULTTUNE may result in it not being set at all for the multilib, leaving the TUNE default to the same as the "normal" DEFAULTTUNE.  This condition causes strange behavior in the system.

Suggested sanity checks:

(normal) DEFAULTTUNE != multilib DEFAULTTUNE
(normal) MACHINE != multilib MACHINE
MULTILIB arch is defined