target gdb: depend on thread support

Compiling gdb for the target requires thread support in the C library,
otherwise:

/home/test/outputs/test-888/toolchain/gdb-7.3.1/gdb/gdb_thread_db.h:37:21: fatal error: pthread.h: No such file or directory

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