backends: Hook abi_cfi for arm.

backends: Hook abi_cfi for arm.

New arm_abi_cfi that defines initial CFA, rules for callee-saved regs,
including VFP ones and return register.

Note the DWARF abi extension for ARM says that "registers intentionally
unused" should also be initialized as if by DW_CFA_same_value.  The
example given is "an integer-only function might be included in one
executable file for targets with VFP and another for targets without".
We don't currently do this yet. See the DWARF and calling convention
documents:

DWARF for the ARM Architecture ABI r2.09
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0040b/IHI0040B_aadwarf.pdf

Procedure Call Standard for the ARM Architecture ABI r2.09
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042e/IHI0042E_aapcs.pdf

Also adds addrcfi arm testcase.

Signed-off-by: Mark Wielaard <mjw@redhat.com>