gdb: properly set the gdb site in all cases

Until now, the gdb site for AVR32 was adjusted only when GDB_VERSION
was not set, i.e only when the cross-gdb is *not* built. We should in
fact also set the site when the cross-gdb is built, so when
GDB_VERSION is defined through package/gdb/Config.in.host.

So, instead, we now have something like:

ifeq ($(GDB_VERSION),)
 # we define here a default version that is used when the cross-gdb is
 # not built and only the target gdb is selected.
endif

ifeq ($(GDB_VERSION),something-special)
GDB_SITE = some-special-site-for-some-special-gdb-version
endif

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