toolchain/external: fix gdbserver install with Linaro 2015.08

In the latest Linaro toolchain, the gdbserver has moved (surprise!)
and is now located side-by-side with the toolchain executables.

This commit adds this path as a new location where to search for a
gdbserver, and while at it wraps the line that has become too long in
the process.

[Thomas: rework commit log according to Yann's suggestion.]

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