is there any reason why you don't build the compilers/gdb with buildroot?
We have some issues running gdb from the toolchain because we have a 64bit system and there are missing dependencies (python2.7 386).
Compiling Buildroot with its own toolchain seems to work with the minimal configuration. After installing the libpython2:i386 package the toolchain provided by F&S also works fine. Maybe we change over to a newer gcc because of C++11 but at the moment we will use 4.7.