toolchain/external: better check for gcc-5

gcc will always report a three-digit version sting, like 4.9.3 or 5.1.0.

For gcc before 5, we want to check the first two digits, while starting
with gcc 5, we are only concerned about the first digit.

So, change our matching code to test for the leading part of the version
string, up to the first dot after as-many version digit we're interested
in.

Note: we're adding the dot in the .mk code rather than in the Kconfig
symbol, because it seemed cleaner to do so.

Reported-by: Jörg Krause <joerg.krause@embedded.rocks>
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>