travis-ci: Switch over to Linaro gcc-6.3.1 toolchains for ARM

Linaro provides a number of pre-built GCC toolchains for both 32 and
64bit ARM.  Switch to their 2017.02 release of gcc-6.3.1 for both.

Cc: Koen Kooi <koen.kooi@linaro.org>
Signed-off-by: Tom Rini <trini@konsulko.com>