Change signature of linux_target_ops.new_thread

Change signature of linux_target_ops.new_thread

This commit changes the signature of linux_target_ops.new_thread in
gdbserver to match that used in GDB's equivalent.

gdb/gdbserver/ChangeLog:

	* linux-low.h (linux_target_ops) <new_thread>: Changed signature.
	* linux-arm-low.c (arm_new_thread): Likewise.
	* linux-aarch64-low.c (aarch64_linux_new_thread): Likewise.
	* linux-mips-low.c (mips_linux_new_thread): Likewise.
	* linux-x86-low.c (x86_linux_new_thread): Likewise.
	* linux-low.c (add_lwp): Update the_low_target.new_thread call.