gdb_server: add "not supported" Z-packet reply

gdb_server: add "not supported" Z-packet reply

GDB remote serial protocol specifies breakpoint/watchpoint packet
responses can be an empty string to indicate the specified breakpoint
type is not supported.  Add support for this response alongside existing
"OK", "E NN" replies.

Signed-off-by: Ian Thompson <ianst@cadence.com>
Change-Id: Iaf6280e4c936eb95a92bc80cc74d451ebb328dc3
Reviewed-on: https://review.openocd.org/c/openocd/+/7102
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>