Use debug_printf to print debug message

Use debug_printf to print debug message

Some functions in aarch64-linux-nat.c and linux-aarch64-low.c looks
the same except for the code printing debug message.  In GDB, we use
fprintf_unfiltered (gdb_stdlog, ...) while in GDBserver, we use
fprintf (stderr, ...).  This patch is to change them to use debug_printf
so that these functions are the same, and I can move them to a common
place in the following patch.

gdb:

2015-07-17  Yao Qi  <yao.qi@linaro.org>

	* aarch64-linux-nat.c (aarch64_show_debug_reg_state): Use
	debug_printf.
	(aarch64_handle_unaligned_watchpoint): Likewise.

gdb/gdbserver:

2015-07-17  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (aarch64_show_debug_reg_state): Use
	debug_printf.
	(aarch64_handle_unaligned_watchpoint): Likewise.