config: add option to build multilib

Add a toolchain-wide option to enable multilib.

This is currently a noop, and will be implemented
in subsequent patches for each impacted components.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 This is NOT the official version of dialog.  This version has been

 significantly modified from the original.  It is for use by the Linux

 kernel configuration script.  Please do not bother Savio Lam with 

 questions about this program.