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>

 # Alpha specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORTS_WITH_CPU

 ## select ARCH_SUPPORTS_WITH_TUNE

 ##

 ## help The Alpha architecture.