arch/arm: some variants need different gcc versions

arch/arm: some variants need different gcc versions

Take the conditions currently specified in the gcc version choice.

Also, the conditions explained in the commit log for 78c2a9f7 were not
all properly applied, especially the a57-a53 combo needs gcc-6, but
78c2a9f7 forgot to add the condition to gcc-4.9.

gcc-4.9 was excluded for cortex-a17 and a72, but the CodeSourcery
external toolchain, which uses 4.8, was not excluded for those two
cores. Now it is.

Remove the arch condition from gcc and the external toolchains.

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