Fix -exec-run not running asynchronously with mi-async on (PR gdb/18077)

Fix -exec-run not running asynchronously with mi-async on (PR gdb/18077)

When doing -exec-run on a freshly started GDB, the only target on the
target stack at the time the dummy one.  When mi_async_p is called to
know whether the run should be async, it queries whether the current
target (dummy) supports async, and the answer is no.  The fix is to make
the code query the target that will be used for the run, which is not
necessarily the current target.

No regressions in the gdb.mi directory using the unix, native-gdbserver
and native-extended-gdbserver boards.  The test doesn't pass when
forcing maint set target-async off, obviously, since it makes mi-async
have no effect.  It doesn't seem like other tests are checking for that
eventuality, so I didn't in the new test.

gdb/ChangeLog:

	* mi/mi-main.c (run_one_inferior): Use run target to determine
	whether to run async or not.
	(mi_cmd_exec_run): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-async-run.exp: New file.
	* gdb.mi/mi-async-run.c: New file.