Fix mi_gdb_exit with secondary MI channels

Fix mi_gdb_exit with secondary MI channels

Tests that use a secondary MI channel (i.e., either tests that call
mi_gdb_start with separate-mi-tty, or all tests when
FORCE_SEPARATE_MI_TTY=1 is specified on the make check command line),
don't close GDB correctly.

E.g., if you run gdb.mi/mi-exec-run.exp in a loop:

  while true; do make check TESTS="gdb.mi/mi-exec-run.exp"; done

you can see more than one gdb running at the same time:

  $ ps -ef | grep -v grep | grep "gdb/gdb"
  pedro      40507       1  7 15:47 ?        00:00:00 /home/pedro/gdb/build/gdb/testsuite/../../gdb/gdb -nw -nx -data-directory /home/pedro/gdb/build/gdb/testsuite/../data-directory
  pedro      40562       1  0 15:47 ?        00:00:00 /home/pedro/gdb/build/gdb/testsuite/../../gdb/gdb -nw -nx -data-directory /home/pedro/gdb/build/gdb/testsuite/../data-directory
  pedro      40727       1  0 15:47 ?        00:00:00 /home/pedro/gdb/build/gdb/testsuite/../../gdb/gdb -nw -nx -data-directory /home/pedro/gdb/build/gdb/testsuite/../data-directory
  pedro      40786       1  0 15:47 ?        00:00:00 /home/pedro/gdb/build/gdb/testsuite/../../gdb/gdb -nw -nx -data-directory /home/pedro/gdb/build/gdb/testsuite/../data-directory

This commit fixes it.

gdb/testsuite/ChangeLog:

	* lib/mi-support.exp (mi_uncatched_gdb_exit) Switch to the main
	spawn_id before calling remote_close.  Close secondary MI channel.