Make default_mi_gdb_start/dbx_gdb_start use gdb_spawn

Make default_mi_gdb_start/dbx_gdb_start use gdb_spawn

If a board file wants to customize how gdb is launched, the obvious
way is to have the board override gdb_spawn.  However, that doesn't
work for either gdb.mi/ testcases or gdb.base/dbx.exp, because
default_mi_gdb_start and dbx_gdb_start don't use gdb_spawn currently.

That is fixed by this patch.

gdb/testsuite/

	* gdb.base/dbx.exp (dbx_gdb_start): Adjust to use gdb_spawn
	instead of spawning GDB with remote_spawn.
	* lib/mi-support.exp (default_mi_gdb_start): Adjust to use
	gdb_spawn instead of spawning GDB with remote_spawn.