external-sourcery: extract src and dest sysroot paths from gcc

external-sourcery: extract src and dest sysroot paths from gcc

Rather than hardcoding the multilib path in a map, and hardcoding dest sysroot
symlink creation in a hook, now we just use -print-sysroot for both, and pass
the appropriate multilib args to the toolchain for particular tunes.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>