Remove support for thread events without PTRACE_EVENT_CLONE in GDBServer.

Remove support for thread events without PTRACE_EVENT_CLONE in GDBServer.

This patch removes support for thread events if PTRACE_EVENT_CLONE is not
supported in GDBServer.

Before, on systems that did not support PTRACE_EVENT_CLONE, both GDB and
GDBServer coordinated with libthread_db.so to insert breakpoints at magic
locations in libpthread.so, in order to break at thread creation and thread
death.

Simple software single stepping support was implemented to step over these
breakpoints in case there was no hardware single stepping support. However,
these simple software single stepping implementations were not fit for any other
use as discussed in :
https://sourceware.org/ml/gdb-patches/2015-04/msg01110.html

These too simple implementations conflict with ongoing work to make proper
implementations of software single stepping in GDBServer.

The problem is that if some implementations are correct and others are not and
only there for the thread magic breakpoint, we can't enable features based
solely software single step support since some would be broken.

To keep the incorrect implementations and allow the new proper ones at the same
time we would need to implement fallback code and it quickly becomes ugly and
confusing with multiple checks for legacy software single step or proper
software single step.

However, PTRACE_EVENT_CLONE was first introduced in Linux 2.5.46,
released in November 2002.

So I think it's reasonable to just remove support for kernels that don't support
PTRACE_EVENT_CLONE, and sidestep the libthread_db breakpoints issues entirely.

This thread on the mailling list discusses the issue :
https://sourceware.org/ml/gdb/2015-10/msg00078.html

No regressions, tested on ubuntu 14.04 ARMv7 and x86.
With gdbserver-{native,extended} / { -marm -mthumb }

gdb/gdbserver/ChangeLog:

	* linux-low.c (linux_look_up_symbols): Don't call
	linux_supports_traceclone.
	* linux-low.h (thread_db_init): Remove use_events argument.
	* thread-db.c (thread_db_use_event): Remove global variable.
	(struct thread_db) <td_thr_event_enable_p>: Remove field.
	(struct thread_db) <td_create_bp>: Remove field.
	(thread_db_create_event): Remove function.
	(thread_db_enable_reporting): Likewise.
	(find_one_thread): Don't check for thread_db_use_events.
	(attach_thread): Likewise.
	(thread_db_load_search): Remove td_thr_event_enable_p initialization.
	(try_thread_db_load_1): Don't check for thread_db_use_events.
	(thread_db_init): Remove use_events argument and thread events
	handling.
	(remove_thread_event_breakpoints): Remove function.
	(thread_db_detach): Remove call to remove_thred_event_breakpoints.