Test different follow-exec-mode settings in gdb.multi/multi-arch-exec.exp

Test different follow-exec-mode settings in gdb.multi/multi-arch-exec.exp

Using follow-exec-mode "new" takes a different code path than "same", so
it's interesting to test this path in combination with a change in
architecture of the inferior.  This test fails if you remove the
previous patch.

gdb/testsuite/ChangeLog:

	* gdb.multi/multi-arch-exec.exp: Test with different
	"follow-exec-mode" settings.
	(do_test): New procedure.