[ARM] Make thumb2_breakpoint static again

[ARM] Make thumb2_breakpoint static again

This patch makes thumb2_breakpoint static.  When writing this patch,
I find the only reason we keep thumb2_breakpoint extern is that it
is used as an argument passed to arm_gdbserver_get_next_pcs.  However,
field arm_thumb2_breakpoint is only used in a null check in
thumb_get_next_pcs_raw, so I wonder why do need to pass thumb2_breakpoint
to arm_gdbserver_get_next_pcs.

thumb2_breakpoint was added by Daniel Jacobowitz in order to support
single-step IT block
https://sourceware.org/ml/gdb-patches/2010-01/msg00624.html  the logic
there was if we have 32-bit thumb-2 breakpoint defined, we can safely
single-step IT block, otherwise, we can't.  Daniel didn't want to use
16-bit thumb BKPT instruction, because it triggers even on instruction
which should be executed.  Secondly, using 16-bit thumb illegal
instruction on top of 32-bit thumb instruction may break the meaning of
original IT blocks, because the other 16-bit can be regarded as an
instruction.  See more explanations from Daniel's kernel patch
http://www.spinics.net/lists/arm-kernel/msg80476.html

Let us back to this patch, GDB/GDBserver can safely single step
IT block if thumb2_breakpoint is defined, but the single step logic
doesn't have to know the thumb-2 breakpoint instruction.  Only
breakpoint insertion mechanism decides to use which breakpoint
instruction.  In the software single step code, instead of pass
thumb2_breakpoint, we can pass a boolean variable
has_thumb2_breakpoint indicate whether the target has thumb-2
breakpoint defined, which is equivalent to the original code.

Regression tested on arm-linux.  No regression.

gdb:

2016-01-14  Yao Qi  <yao.qi@linaro.org>

	* arch/arm-get-next-pcs.c (arm_get_next_pcs_ctor): Change
	argument arm_thumb2_breakpoint to has_thumb2_breakpoint.
	(thumb_get_next_pcs_raw): Check has_thumb2_breakpoint
	instead.
	* arch/arm-get-next-pcs.h (struct arm_get_next_pcs)
	<arm_thumb2_breakpoint>: Remove.
	<has_thumb2_breakpoint>: New field.
	(arm_get_next_pcs_ctor): Update declaration.
	* arm-linux-tdep.c (arm_linux_software_single_step): Pass
	1 to arm_get_next_pcs_ctor.
	* arm-tdep.c (arm_software_single_step): Pass 0 to
	arm_get_next_pcs_ctor.

gdb/gdbserver:

2016-01-14  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch32-low.c (thumb2_breakpoint): Make it static.
	* linux-aarch32-low.h (thumb2_breakpoint): Remove declaration.
	* linux-arm-low.c (arm_gdbserver_get_next_pcs): Pass 1 to
	arm_get_next_pcs_ctor.