gdbserver: Ensure all debug output uses debug functions

gdbserver: Ensure all debug output uses debug functions

All debug output needs to go via debug functions to ensure it writes to the
correct output stream.

gdb/ChangeLog:

	* nat/linux-waitpid.c (linux_debug): Call debug_vprintf.

gdb/gdbserver/ChangeLog:

	* ax.c (ax_vdebug): Call debug_printf.
	* debug.c (debug_write): New function.
	* debug.h (debug_write): New declaration.
	* linux-low.c (sigchld_handler): Call debug_write.