kernel: remove gdb_server

This is unmaintained and currently has no known users. It was
added to support a Wind River project. If in the future we need it
again, we should re-introduce it with an exception-based mechanism
for catching out-of-bounds memory queries from the debugger.

The mem_safe subsystem is also removed, it is only used by the
GDB server. If its functionality is needed in the future, it
shoudl be replaced with an exception-based mechanism.

The _image_{ram, rom, text}_{start, end} linker variables have
been left in place, they will be re-purposed and expanded to
support memory protection.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>