Implement 'catch syscall' for gdbserver

This adds a new QCatchSyscalls packet to enable 'catch syscall', and new
stop reasons "syscall_entry" and "syscall_return" for those events.  It
is currently only supported on Linux x86 and x86_64.

gdb/ChangeLog:

2016-01-12  Josh Stone  <jistone@redhat.com>
	    Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* NEWS (Changes since GDB 7.10): Mention QCatchSyscalls and the
	syscall_entry and syscall_return stop reasons.  Mention GDB
	support for remote catch syscall.
	* remote.c (PACKET_QCatchSyscalls): New enum.
	(remote_set_syscall_catchpoint): New function.
	(remote_protocol_features): New element for QCatchSyscalls.
	(remote_parse_stop_reply): Parse syscall_entry/return stops.
	(init_remote_ops): Install remote_set_syscall_catchpoint.
	(_initialize_remote): Config QCatchSyscalls.
	* linux-nat.h (struct lwp_info) <syscall_state>: Comment typo.

gdb/doc/ChangeLog:

2016-01-12  Josh Stone  <jistone@redhat.com>
	    Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.texinfo (Remote Configuration): List the QCatchSyscalls packet.
	(Stop Reply Packets): List the syscall entry and return stop reasons.
	(General Query Packets): Describe QCatchSyscalls, and add it to the
	table and the detailed list of stub features.

gdb/gdbserver/ChangeLog:

2016-01-12  Josh Stone  <jistone@redhat.com>
	    Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* inferiors.h: Include "gdb_vecs.h".
	(struct process_info): Add syscalls_to_catch.
	* inferiors.c (remove_process): Free syscalls_to_catch.
	* remote-utils.c (prepare_resume_reply): Report syscall_entry and
	syscall_return stops.
	* server.h (UNKNOWN_SYSCALL, ANY_SYSCALL): Define.
	* server.c (handle_general_set): Handle QCatchSyscalls.
	(handle_query): Report support for QCatchSyscalls.
	* target.h (struct target_ops): Add supports_catch_syscall.
	(target_supports_catch_syscall): New macro.
	* linux-low.h (struct linux_target_ops): Add get_syscall_trapinfo.
	(struct lwp_info): Add syscall_state.
	* linux-low.c (handle_extended_wait): Mark syscall_state as an entry.
	Maintain syscall_state and syscalls_to_catch across exec.
	(get_syscall_trapinfo): New function, proxy to the_low_target.
	(linux_low_ptrace_options): Enable PTRACE_O_TRACESYSGOOD.
	(linux_low_filter_event): Toggle syscall_state entry/return for
	syscall traps, and set it ignored for all others.
	(gdb_catching_syscalls_p): New function.
	(gdb_catch_this_syscall_p): New function.
	(linux_wait_1): Handle SYSCALL_SIGTRAP.
	(linux_resume_one_lwp_throw): Add PTRACE_SYSCALL possibility.
	(linux_supports_catch_syscall): New function.
	(linux_target_ops): Install it.
	* linux-x86-low.c (x86_get_syscall_trapinfo): New function.
	(the_low_target): Install it.

gdb/testsuite/ChangeLog:

2016-01-12  Josh Stone  <jistone@redhat.com>
	    Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.base/catch-syscall.c (do_execve): New variable.
	(main): Conditionally trigger an execve.
	* gdb.base/catch-syscall.exp: Enable testing for remote targets.
	(test_catch_syscall_execve): New, check entry/return across execve.
	(do_syscall_tests): Call test_catch_syscall_execve.

gdb/linux-nat.h

@@ -88,7 +88,7 @@ struct lwp_info
      or to a local variable in lin_lwp_wait.  */
   struct target_waitstatus waitstatus;
 
-  /* Signal wether we are in a SYSCALL_ENTRY or
+  /* Signal whether we are in a SYSCALL_ENTRY or
      in a SYSCALL_RETURN event.
      Values:
      - TARGET_WAITKIND_SYSCALL_ENTRY