toolchain/external: use -dumpversion to check gcc version

toolchain/external: use -dumpversion to check gcc version

Currently, `--version` option is used and later matched with a regex to get
the actual gcc version.  There's a dedicated gcc option to do exactly that:
`-dumpversion`.

Also `--version` may return a string customised by a vendor that provides
the toolchain, which makes the current regex approach error prone.  In
fact, this situation has been seen with a real customised toolchain.

Signed-off-by: Krzysztof Konopko <kris@youview.com>
Signed-off-by: Tomasz Szkutkowski <tomasz.szkutkowski@youview.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>