toolchain-external: add Sourcery MIPS 2013.11, remove Sourcery MIPS 2012.03

This commit adds the support for the recently release Sourcery MIPS
2013.11 toolchain (gcc 4.8, gdb 7.6, glibc 2.18), and consequently
removes the support for the Sourcery MIPS 2012.03 toolchain.

While we're at it, also fix the incorrect help text related to the
MIPS64 multilib selection.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>