toolchain/external: bump Linaro AArch64 to 2015.08

That toolchain is built for an x86_64 host, so we make it available only
for x86_64, and we keep the old 2014.09 toolchain for x86 hosts.

To avoid dealing with legacy symbols and introduce versioned options,
we reuse the same symbol for both toolchains. Thanks to the different
depednencies (on the host), we can give them different prompts and
different help texts.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>