toolchain-external: add support for gcc version dependency

This commit wires up the gcc version dependency mechanism in the
external toolchain backend. To do so, it:

 * Changes the definition of all pre-defined external toolchain
   profiles to select the appropriate BR2_TOOLCHAIN_GCC_AT_LEAST_*
   option.

 * For custom external toolchains, provides a visible Config.in
   "choice" to select the gcc version used in the external toolchain.

 * Adds a new check_gcc_version function, that verifies that the real
   gcc version found in the external toolchain matches the one
   declared in the Buildroot configuration.

[Thomas: use better sed expression proposed by Yann E. Morin, which
works with more cases.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>