arm: only compile gdb stubs when CONFIG_GDB_INFO=y

They use data that is unavailable to the unified kernel, and are not
used anymore really. For now, only compile them when CONFIG_GDB_INFO=y,
and never enable CONFIG_GDB_INFO when building the unified kernel.

These files should go away when the unified kernel is made the only
kernel type.

Change-Id: I0a2a917dd453ecaae729125008756e0f676df16d
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>