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>

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."