toolchain/gcc: fix broken gcc version selection

toolchain/gcc: fix broken gcc version selection

Config evaluation require default with if to be put before the generic
default config with no condition. Putting the default config before any
conditional default results in always selecting the non conditional one.

This results in the version be hardcoded to gcc 12 even if gcc 11 is
selected in the Advanced build options.

Fix this by putting the gcc 12 default option as last after ANY
conditional default config.

Fixes: d9de5252a44e ("toolchain/gcc: switch to version 12 by default")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>