gdb_server: Operate on available targets.

gdb_server: Operate on available targets.

When SMP is enabled, gdb will always use the first target in the SMP
group. That doesn't work when that first target is unavailable, but
others in the SMP group are still available.

For cases where gdb expects an operation to affect the entire group (run
control, memory access), find the first available target in an SMP group
and use that.

Imported from
https://github.com/riscv-collab/riscv-openocd/pull/767

Change-Id: I4bed600da3ac0fdfe4287d8fdd090a58452db501
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8912
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>