buildroot

gdb: enable simulator support for host build

Add an option to enable GDB simulator support, so
it can be used to test some no-MMU systems.

Newer GDB Blackfin Simulator need a workaround for in-tree compile.
It doesn't break for older Buildroot supported GDB Blackfin
Simulator.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>