Refactor arm_software_single_step to use regcache

Refactor arm_software_single_step to use regcache

This patch is in preparation for software single step support on ARM in
GDBServer. It refactors arm_*_software_single_step and sub-functions to
use regcache instead of frame to access registers so that the code can be
shared more easily between GDB and GDBServer.

Note also that since the intention is at some point to get rid of frame
completely in that function, memory reads have also been replaced by
read_memory_unsigned_integer rather than get_frame_memory_unsigned.

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

gdb/ChangeLog:

	* arm-linux-tdep.c (arm_linux_sigreturn_next_pc_offset): New function.
	(arm_linux_sigreturn_next_pc): Likewise.
	(arm_linux_syscall_next_pc): Use regcache instead of frame.
	(arm_linux_software_single_step): Likewise.
	* arm-tdep.c (arm_is_thumb): New function.
	(shifted_reg_va): Use regcache instead of frame.
	(thumb_get_next_pc_raw): Likewise.
	(arm_get_next_pc_raw): Likewise.
	(arm_get_next_pc): Likewise.
	(thumb_deal_with_atomic_sequence_raw): Likewise.
	(arm_deal_with_atomic_sequence_raw): Likewise.
	(arm_deal_with_atomic_sequence): Likewise.
	(arm_software_single_step): Likewise.
	* arm-tdep.h (struct gdbarch_tdep): Use regcache for syscall_next_pc.
	(arm_get_next_pc): Use regcache.
	(arm_deal_with_atomic_sequence): Likewise.
	(arm_is_thumb): New declaration.
	* regcache.c (regcache_raw_get_unsigned): New function.
	* regcache.h (regcache_raw_get_unsigned): New function declaration.