Don't call set_gdbarch_cannot_step_breakpoint in aarch64_gdbarch_init

Don't call set_gdbarch_cannot_step_breakpoint in aarch64_gdbarch_init

Hi,
I am looking at the following fails in aarch64-linux,

stepi^M
47        NOP; /* after permanent bp */^M
(gdb) FAIL: gdb.base/bp-permanent.exp: always_inserted=off, sw_watchpoint=0: stepi signal with handler: single-step to handler

the test expects GDB single step into signal handler, but GDB doesn't.
The code in infrun.c:resume

      /* Most targets can step a breakpoint instruction, thus
	 executing it normally.  But if this one cannot, just
	 continue and we will hit it anyway.  */
      if (gdbarch_cannot_step_breakpoint (gdbarch))
	step = 0;

change the intended action from "step" to "continue".  The gdbarch method
cannot_step_breakpoint isn't documented well, and I don't get much clue
after explore the history.  However, from the comments above,
aarch64-linux can step a breakpoint instruction, so don't need to call
set_gdbarch_cannot_step_breakpoint.

gdb:

2015-03-20  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (aarch64_gdbarch_init): Don't call
	set_gdbarch_cannot_step_breakpoint.