Disable Z0 packet on aarch64 on multi-arch debugging

Disable Z0 packet on aarch64 on multi-arch debugging

In multi-arch debugging, if GDB sends Z0 packet, GDBserver should be
able to do several things below:

 - choose the right breakpoint instruction to insert according to the
   information available, such as 'kind' in Z0 packet and address,

 - choose the right breakpoint instruction to check memory writes and
   validate inserted memory breakpoint

 - be aware of different breakpoint instructions in $ARCH_breakpoint_at.

unfortunately GDBserver can't do them now.  Although x86 GDBserver
supports multi-arch, it doesn't need to support them above because
breakpoint instruction on i686 and x86_64 is the same.  However,
breakpoint instructions on aarch64 and arm (arm mode, thumb1, and thumb2)
are different.

I tried to teach aarch64 GDBserver backend to be really
multi-arch-capable in the following ways,

 - linux_low_target return the right breakpoint instruction according to
   the 'kind' in Z0 packet, and insert_memory_breakpoint can do the right
   thing.
 - once breakpoint is inserted, the breakpoint data and length is recorded
   in each breakpoint object, so that validate_breakpoint and
   check_mem_write can get the right breakpoint instruction from each
   breakpoint object, rather than from global variable breakpoint_data.
 - linux_low_target needs another hook function for pc increment after
   hitting a breakpoint.
 - let set_breakpoint_at, which is widely used for tracepoint, use the
   'default' breakpoint instruction.  We can always use aarch64 breakpoint
   instruction since arm doesn't support tracepoint yet.

looks it is not a small piece of work, so I decide to disable Z0 packet
on multi-arch, which means aarch64 GDBserver only supports Z0 packet
if it is started to debug only one process (extended protocol is not
used) and process target description is 64-bit.

gdb/gdbserver:

2015-08-04  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (aarch64_supports_z_point_type): Return
	0 for Z_PACKET_SW_BP if it may be used in multi-arch debugging.
	* server.c (extended_protocol): Remove "static".
	* server.h (extended_protocol): Declare it.