scripts/ext-toolchain: implement external GCC version detection

scripts/ext-toolchain: implement external GCC version detection

Some package may needs to enable compatibility option based on the GCC
version.

Currently the GCC version is set based on the default value and doesn't
actually reflect the real value provided by the external toolchain if
used.

Fix this by correctly detecting the GCC version in the external
toolchain and set the correct value in CONFIG_GCC_VERSION.

A new option is added in menuconfig to manually set the GCC version if
needed.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>