package/gdb: ARC version has stringent dependencies on host gcc

package/gdb: ARC version has stringent dependencies on host gcc

The ARC gdb version requires a gcc >= 4.8 (because C++11).

Since the host variant uses the same sources as the target variant, we
must ensure that we have a host gcc >= 4.8 as well.

Move the architecture dependencies to their own symbol, and also hide
the comment when they are not met.

Fixes:
    http://autobuild.buildroot.net/results/43eae264991aa369490236c7bd59c0b6a67fcf25/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>