package/gdb: rework dependency for C++11

package/gdb: rework dependency for C++11

As we are about to switch to 8.0 as the default gdb version, we
need to adjust how the gdb dependencies are handled. Indeed, from 8.0
onwards, gdb needs a C++11 capable compiler, i.e at least gcc 4.8.

Until now, Config.in.host was making sure that gdb 8.0 was not
selectable if the cross-compilation toolchain did not have C++ support
with gcc >= 4.8. This worked fine because the default version of gdb,
used as the target gdb version when no host gdb is built, was 7.11,
and did not require C++11.

With the switch to 8.0 as the default version, when target gdb is
enabled but not host gdb, 8.0 is used, which means we need a C++11
capable compiler. The dependencies in Config.in.host are no longer
sufficient.

So instead, we remove the target-related dependencies from
Config.in.host and move them properly to Config.in. The overall logic
is the following:

 - In Config.in.host, BR2_PACKAGE_HOST_GDB_ARCH_SUPPORTS ensures that
   we have at least host gcc 4.8 if we're on ARC, because the ARC gdb
   needs C++11. We remove the target toolchain related dependencies
   from here.

 - In Config.in.host, the version selection ensures that 8.0 cannot be
   selected if the host toolchain does not have at least gcc 4.8. We
   remove the target toolchain related dependencies from here.

 - In Config.in.host, we introduce a BR2_PACKAGE_GDB_NEEDS_CXX11
   option, that indicates whether the currently selected version of
   gdb requires C++11 support in the toolchain to build the target
   variant. Even though this option is more related to the target
   variant of gdb, we keep it in Config.in.host so that it appears
   next to the definition of BR2_GDB_VERSION, to make sure they are
   kept in sync.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>