dwarf2cfi: Handle return column save from CIE.

dwarf2cfi: Handle return column save from CIE.

When we record a save of the return column in the CIE, remember
that while processing the FDE.  This requires propagating the
handling of PC_RTX as a representative of the return column to
more locations.

MIPS had been handling this case by hand, and is no longer required.

        * dwarf2cfi.c (cie_return_save): New.
        (queue_reg_save): Use compare_reg_or_pc.
        (dwarf2out_flush_queued_reg_saves): Handle pc_rtx as return column.
        (dwarf2out_frame_debug_expr): Likewise.
        (dwarf2out_frame_debug_cfa_register): Record saved reg for pc too.
        (initial_return_save): Likewise.
        (execute_dwarf2_frame): Save and restore initial return save from
        the cie to the fde.
        * config/mips/mips.c (mips_frame_set): Remove special case for
        DWARF_FRAME_RETURN_COLUMN.

From-SVN: r176099