gdb/mi: fix regression in mi -add-inferior command

Prior to the multi-target support commit:

  commit 5b6d1e4f
  Date:   Fri Jan 10 20:06:08 2020 +0000

      Multi-target support

When a new inferior was added using the MI -add-inferior command, the
new inferior would be using the same target as all the other
inferiors.  This makes sense, GDB only supported a single target stack
at a time.

After the above commit, each inferior has its own target stack.

To maintain backward compatibility, for the CLI add-inferior command,
when a new inferior is added the above commit has the new inferior
inherit a copy of the target stack from the current inferior.

Unfortunately, this same backward compatibility is missing for the MI.

This commit fixes this oversight.

Now, when the -add-inferior MI command is used, the new inferior will
inherit a copy of the target stack from the current inferior.