linux_nat_target: More low methods
    
    This converts the remaining linux-nat.c hooks low_ methods like had
    been started in a previous patch.  The linux_nat_set_foo routines are
    all gone with this.
    
    gdb/ChangeLog:
    yyyy-mm-dd  Pedro Alves  <palves@redhat.com>
    
    	* linux-nat.h (linux_nat_target) 
    	low_delete_thread, low_new_fork, low_forget_process,
    	low_prepare_to_resume, low_siginfo_fixup, low_status_is_event>:
    	New virtual methods.
    	(linux_nat_set_new_thread, linux_nat_set_delete_thread)
    	(linux_nat_new_fork_ftype, linux_nat_set_new_fork)
    	(linux_nat_forget_process_ftype, linux_nat_set_forget_process)
    	(linux_nat_forget_process, linux_nat_set_siginfo_fixup)
    	(linux_nat_set_prepare_to_resume, linux_nat_set_status_is_event):
    	Delete.
    	* linux-fork.c (delete_fork): Adjust to call low method.
    	* linux-nat.c (linux_nat_new_thread, linux_nat_delete_thread)
    	(linux_nat_new_fork, linux_nat_forget_process_hook)
    	(linux_nat_prepare_to_resume, linux_nat_siginfo_fixup)
    	(linux_nat_status_is_event):
    	(linux_nat_target::follow_fork, lwp_free, add_lwp, detach_one_lwp)
    	(linux_resume_one_lwp_throw, linux_handle_extended_wait): Adjust
    	to call low method.
    	(sigtrap_is_event): Rename to ...
    	(linux_nat_target::low_status_is_event): ... this.
    	(linux_nat_set_status_is_event): Delete.
    	(save_stop_reason, linux_nat_wait_1)
    	(linux_nat_target::mourn_inferior, siginfo_fixup): Adjust to call
    	low methods.
    	(linux_nat_set_new_thread, linux_nat_set_delete_thread)
    	(linux_nat_set_new_fork, linux_nat_set_forget_process)
    	(linux_nat_forget_process, linux_nat_set_siginfo_fixup)
    	(linux_nat_set_prepare_to_resume): Delete.
    	* aarch64-linux-nat.c: All linux_nat_set_* callbacks converted to
    	low virtual methods.
    	* amd64-linux-nat.c: Likewise.
    	* arm-linux-nat.c: Likewise.
    	* i386-linux-nat.c: Likewise.
    	* ia64-linux-nat.c: Likewise.
    	* mips-linux-nat.c: Likewise.
    	* ppc-linux-nat.c: Likewise.
    	* s390-linux-nat.c: Likewise.
    	* sparc64-linux-nat.c: Likewise.
    	* x86-linux-nat.c: Likewise.
    	* x86-linux-nat.h: Include "nat/x86-linux.h".
    	(x86_linux_nat_target) 
    	low_prepare_to_resume, low_new_thread, low_delete_thread>:
    	Override methods.