Refactor the breakpoint definitions in linux-arm-low.c.

Refactor the breakpoint definitions in linux-arm-low.c.

Before arm_sw_breakpoint_from_kind would use an #ifdef to return the right
arm_breakpoint from the abi or eabi breakpoint type.

arm_breakpoint_at would also check for the arm_breakpoint ||
arm_eabi_breakpoint.

Thus the selected arm_breakpoint would be what arm_sw_breakpoint_from_kind
returned and arm_breakpoint was arm_abi_breakpoint.

This patch makes it more clear by naming those for what they are : 2 separate
entities: arm_abi_breakpoint and arm_eabi_breakpoint and set the current used
one as arm_breakpoint.

This allows a cleaner arm_sw_breakpoint_from_kind as it just returns
arm_breakpoint rather than having the #ifdef in that function.

Any other reference to the arm_breakpoint can now also be clear of #ifdefs...

No regressions on Ubuntu 14.04 on ARMv7 and x86.
With gdbserver-{native,extended} / { -marm -mthumb }

gdb/gdbserver/ChangeLog:

	* linux-arm-low.c: Refactor breakpoint definitions.
	(arm_breakpoint_at): Adjust for arm_abi_breakpoint.
	(arm_sw_breakpoint_from_kind): Adjust for arm_breakpoint.