gdb: adjust behavior when thread debug is not available

When an external toolchain without thread debug is used, the gdb
package can be selected, but no version can be choosen, since none
match any of the requirements. This leads Buildroot to try to build
gdb for the target without a version being defined, as in the
following build log:

 http://autobuild.buildroot.org/results/84e8fd2df0cc22448052a572c2e9a6e03dd137eb/build-end.log



To fix this, we adjust the dependencies of the BR2_PACKAGE_GDB option
so that the package as a whole is not selectable when the required
conditions are not met. Basically, we have the choice of:

 * Having a toolchain that supports thread debugging, which is needed
   for gdb >= 7.x

 * Having BR2_DEPRECATED enabled, which allows gdb 6.8 to be selected,
   which doesn't require thread debugging

 * Using bfin, since this architectures has a special old gdb version

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>