This works around some side effects of the -rtos hack, namely that we
were unable to set hardware breakpoints on harts whose misa differed
from the first one. There may be other bugs like this one lurking
elsewhere. The only proper solution is for gdb to have a better user
interface when talking to a server that exposes multiple targets, but
that's a very big project.
This fixes#194.
Change-Id: I81aedddeaa922d220e936730e9c731545953ae21
6030644a9d