[gdb/testsuite] Log wait status on process no longer exists error

[gdb/testsuite] Log wait status on process no longer exists error

Proc gdb_test_multiple can run into a process no longer exists error, but when
that happens it shows no details about the process:
...
ERROR: Process no longer exists
...

Fix this by showing the wait status of the process in the log:
...
ERROR: GDB process no longer exists
GDB process exited with wait status 8106 exp8 0 0 CHILDKILLED SIGSEGV \
  {segmentation violation}
...

In order to run the wait commmand we need an explicit pid, so we can't use
any_spawn_id, and duplicate the "-i any_spawn_id eof" pattern for gdb_spawn_id,
and add the wait status logging there.

Build and tested on x86_64-linux.

2018-10-24  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_test_multiple): Log wait status on process no
	longer exists error.