Return zero in aarch64_linux_can_use_hw_breakpoint if target doesn't support HW watch...

Return zero in aarch64_linux_can_use_hw_breakpoint if target doesn't support HW watchpoint/breakpoint

Nowadays aarch64_linux_can_use_hw_breakpoint always return one, but it
can be smarter, say, if GDB knows target doesn't support HW watchpoint
or breakpoint because HW watchpoint/breakpoint is disabled in linux
kernel, for example, it can safely return zero.

gdb:

2015-07-23  Yao Qi  <yao.qi@linaro.org>

	* aarch64-linux-nat.c (aarch64_linux_can_use_hw_breakpoint): If
	TYPE is watchpoint, return zero if aarch64_num_wp_regs is zero.
	If TYPE is breakpoint, return zero if arch64_num_bp_regs is zero.