Move duplicated Linux x86 code to nat/x86-linux.c

Move duplicated Linux x86 code to nat/x86-linux.c

This commit moves two identical functions from gdb/x86-linux-nat.c and
gdb/gdbserver/linux-x86-low.c into the shared file gdb/nat/x86-linux.c.

gdb/ChangeLog:

	* nat/x86-linux.h (x86_linux_new_thread): New declaration.
	(x86_linux_prepare_to_resume): Likewise.
	* x86-linux-nat.c (x86_linux_new_thread):
	Moved to nat/x86-linux.c.
	(x86_linux_prepare_to_resume): Likewise.
	* nat/x86-linux.c (x86_linux_new_thread): New function.
	(x86_linux_prepare_to_resume): Likewise.

gdb/gdbserver/ChangeLog:

	* linux-x86-low.c (x86_linux_new_thread): Moved to
	nat/x86-linux.c.
	(x86_linux_prepare_to_resume): Likewise.