Fix for gdb.server/non-existing-program.exp test case

The test checks to make sure GDB exits cleanly if there is
no valid target binary.  Currently, ppc and S390 fail on this
test.  The function target_post_create_inferior () calls
linux_post_create_inferior () which calls the architecture
specific functions s390_arch_setup () and ppc_arch_setup ()
which make ptrace calls	to access the architecture specific
registers.  These ptrace calls fail because the	process	does
not exist causing GDB to exit on error.

This patch checks to see if the initial ptrace (PTRACE_TRACEME, ...)
call returned a status of TARGET_WAITKIND_EXITED indicating the
target has already exited.  If the target has exited, then the
target_post_create_inferior () is not called since there is no
inferior to be setup.  The test	to see if the initial ptrace
call succeeded is done after the ptrace (PTRACE_TRACEME, ...)
call and the wait for the inferior process to stop, assuming
it exists, has occurred.

The patch has been tested on X86 64-bit, ppc64 and s390.  If
fixes the test failures	on ppc64 and s390.  The	test does not
fail on	X86 64-bit.  The patch does not	introduce any additional
regression failures on any of these three platforms.

gdbserver/ChangeLog

2016-09-06  Carl Love  <cel@us.ibm.com>

	* server.c (start_inferior):  Do not call
	function target_post_create_inferior () if the
	inferior process has already exited.