Initialise target descrption after skipping extra traps for --wrapper

Initialise target descrption after skipping extra traps for --wrapper

Nowadays, when --wrapper is used, GDBserver skips extra traps/stops
in the wrapper program, and stops at the first instruction of the
program to be debugged.  However, GDBserver created target description
in the first stop of inferior, and the executable of the inferior
is the wrapper program rather than the program to be debugged.  In
this way, the target description can be wrong if the architectures
of wrapper program and program to be debugged are different.  This
is shown by some fails in gdb.server/wrapper.exp on buildbot.

We are testing i686-linux GDB (Fedora-i686) on an x86_64-linux box
(fedora-x86-64-4) in buildbot, such configuration causes fails in
gdb.server/wrapper.exp like this:

spawn /home/gdb-buildbot-2/fedora-x86-64-4/fedora-i686/build/gdb/testsuite/../../gdb/gdbserver/gdbserver --once --wrapper env TEST=1 -- :2346 /home/gdb-buildbot-2/fedora-x86-64-4/fedora-i686/build/gdb/testsuite/outputs/gdb.server/wrapper/wrapper
Process /home/gdb-buildbot-2/fedora-x86-64-4/fedora-i686/build/gdb/testsuite/outputs/gdb.server/wrapper/wrapper created; pid = 8795
Can't debug 64-bit process with 32-bit GDBserver
Exiting
target remote localhost:2346
localhost:2346: Connection timed out.
(gdb) FAIL: gdb.server/wrapper.exp: setting breakpoint at marker

See https://sourceware.org/ml/gdb-testers/2015-q3/msg01541.html

In this case, program to be debugged ("wrapper") is 32-bit but wrapper
program ("/usr/bin/env") is 64-bit, so GDBserver gets the 64-bit
target description instead of 32-bit.

The root cause of this problem is that GDBserver creates target
description too early, and the rationale of fix could be creating
target description once the GDBserver skips extra traps and inferior
stops at the first instruction of the program we want to debug.  IOW,
when GDBserver skips extra traps, the inferior's tdesc is NULL, and
mywait and its callees shouldn't use inferior's tdesc, so in this
patch, we skip code that requires register access, see changes in
linux_resume_one_lwp_throw and need_step_over_p.

In linux_low_filter_event, if target description isn't initialised and
GDBserver attached the process, we create target description immediately,
because GDBserver don't have to skip extra traps for attach, IOW, it
makes no sense to use --attach and --wrapper together.  Otherwise, the
process is launched by GDBserver, we keep the status pending, and return.

After GDBserver skipped extra traps in start_inferior, we call a
target_ops hook arch_setup to initialise target description there.

gdb/gdbserver:

2015-07-24  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (linux_arch_setup): New function.
	(linux_low_filter_event): If proc->tdesc is NULL and
	proc->attached is true, call the_low_target.arch_setup.
	Otherwise, keep status pending, and return.
	(linux_resume_one_lwp_throw): Don't call get_pc if
	thread->while_stepping isn't NULL.  Don't call
	get_thread_regcache if proc->tdesc is NULL.
	(need_step_over_p): Return 0 if proc->tdesc is NULL.
	(linux_target_ops): Install arch_setup.
	* server.c (start_inferior): Call the_target->arch_setup.
	* target.h (struct target_ops) <arch_setup>: New field.
	(target_arch_setup): New marco.
	* lynx-low.c (lynx_target_ops): Update.
	* nto-low.c (nto_target_ops): Update.
	* spu-low.c (spu_target_ops): Update.
	* win32-low.c (win32_target_ops): Update.