DWARF2 unwind support

This patch will switch the default to use DWARF2 unwinding and exception handling instead of the old setjmp / longjmp.

I have tested both manual stack unwinding using _Unwind_Backtrace and all kinds of different C++ exceptions I can think of.

To support exceptions through signal handlers one need to add linux-unwind.h like other architectures, but I couldn't find any use except in java so I chose to leave it as a TODO.

I suggest building GCC with --disable-sjlj-exceptions but it should not matter.

For this patch to work, please checkout my pull request related to CFI in or1k-src as well.

ChangeLog:

  (TARGET_EXCEPT_UNWIND_INFO) define.

  (or1k_except_unwind_info) use DWARF2 unwind info as default
  but support SJLJ if forced with --enable-sjlj-exceptions.

  * config/or1k/or1k-protos.h:

  (or1k_expand_pic_symbol_ref) removed unneeded ATTRIBUTE_UNUSED.

  (or1k_eh_return_handler_rtx) defined prototype (body in or1k.c).

  (or1k_return_addr_rtx) defined prototype (body in or1k.c).

  * config/or1k/or1k.c:

  (or1k_save_reg_p) save stack pointer when frame pointer is not saved.

  (or1k_save_reg_p) save registers used with eh_return.

  (or1k_expand_epilogue) do not restore link register if we are
  returning from eh_return.

  (or1k_expand_epilogue) apply EH stack adjustment to stack pointer.

  (or1k_eh_return_handler_rtx) eh_return should write to the link register.

  (or1k_return_addr_rtx) in the case of GOT the link register cannot be read
  after the prologue. in this case, use the stored link register from the
  stack frame.

  (or1k_frame_pointer_required) require FP in eh_return and
  when alloca is used.

  (TARGET_FRAME_POINTER_REQUIRED) define.

  * config/or1k/or1k.h:

  (INITIAL_FRAME_POINTER_OFFSET) not used, removed.

  (RETURN_ADDR_RTX) define.

  (EH_RETURN_REGNUM) define. use reg 23.

  (EH_RETURN_DATA_REGNO) define. use reg 25, 27, 29 and 31.

  (EH_RETURN_STACKADJ_RTX) define.

  (EH_RETURN_HANDLER_RTX) define.