[ARM/AArch64] Fix -Werror=unused-const-variable warnings in GDBserver

[ARM/AArch64] Fix -Werror=unused-const-variable warnings in GDBserver

This patch fixes gcc warning when build ARM GDBserver and AArch64
GDBserver,

AArch64 GDBserver:

gdb/gdbserver/linux-aarch32-low.h:36:29: error: 'thumb2_breakpoint' defined but not used [-Werror=unused-const-variable]
 static const unsigned short thumb2_breakpoint[] = { 0xf7f0, 0xa000 };
                             ^
gdb/gdbserver/linux-aarch32-low.h:34:29: error: 'thumb_breakpoint' defined but not used [-Werror=unused-const-variable]
 static const unsigned short thumb_breakpoint = 0xde01;
                             ^
gdb/gdbserver/linux-aarch32-low.h:28:28: error: 'arm_breakpoint' defined but not used [-Werror=unused-const-variable]
 static const unsigned long arm_breakpoint = arm_eabi_breakpoint;
                            ^
cc1: all warnings being treated as errors

ARM GDBserver:

gdb/gdbserver/linux-aarch32-low.h:34:29: error: 'thumb_breakpoint' defined but not used [-Werror=unused-const-variable]
 static const unsigned short thumb_breakpoint = 0xde01;
                             ^~~~~~~~~~~~~~~~

gdb/gdbserver/linux-aarch32-low.h:28:28: error: 'arm_breakpoint' defined but not used [-Werror=unused-const-variable]
 static const unsigned long arm_breakpoint = arm_eabi_breakpoint;
                            ^~~~~~~~~~~~~~

by simply moving these macros and variables to
linux-aarch32-low.c and only declare thumb2_breakpoint in
linux-aarch32-low.h, which is not perfect, and reveals some issues
in recent arm GDBserver software single step changes.  I'll post
follow-up patches.

gdb/gdbserver:

2016-01-06  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch32-low.h (arm_abi_breakpoint): Move to
	linux-aarch32-low.c.
	(arm_eabi_breakpoint, arm_breakpoint): Likewise.
	(arm_breakpoint_len, thumb_breakpoint_len): Likewise.
	(thumb2_breakpoint, thumb2_breakpoint_len): Likewise.
	(thumb2_breakpoint): Declare.
	* linux-aarch32-low.c (arm_abi_breakpoint): Moved from
	linux-aarch32-low.h.
	(arm_eabi_breakpoint, arm_breakpoint): Likewise.
	(arm_breakpoint_len, thumb_breakpoint_len): Likewise.
	(thumb2_breakpoint, thumb2_breakpoint_len): Likewise.