target/breakpoints: Clear software breakpoints from available targets

If a target where a software breakpoint was set is not currently
available, but there are other targets in the same SMP group that are
available, then we can use those to remove the software breakpoint.

Change-Id: I9faa427c7b3aee31504e6e6599539e6f29b58d8f
Signed-off-by: Tim Newsome <tim@sifive.com>