[PING#2, ARM] fix .cfi inconsistency out of builtin_eh_return

[Olivier Hainque]

Hello,

Ping # 2 for https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00271.html

       * config/arm/arm.c (arm_set_return_address): Use MEM_VOLATILE_P
       on the target mem instead of RTX_FRAME_RELATED_P on the insn to
       prevent DSE.
       (thumb_set_return_address): Likewise.

This one fixes ICEs during the build of libgcc with dwarf2 unwind info
activated. The explanation message is longish but the patch is short.

Thanks in advance,

With Kind Regards,

Olivier