gas: Make dwarf2dbg.c versions specific and add DW_AT_high_pc case for DWARF 4+.

gas: Make dwarf2dbg.c versions specific and add DW_AT_high_pc case for DWARF 4+.

    * dwarf2dbg.c (DWARF2_ARANGES_VERSION): New define to 2.
    (DWARF2_LINE_VERSION): Likewise.
    (out_debug_line): Use DWARF2_LINE_VERSION not DWARF2_VERSION.
    (out_debug_aranges): Use DWARF2_ARANGES_VERSION not DWARF2_VERSION.
    (out_debug_abbrev): Use DW_FORM_data for DW_AT_high_pc when
    DWARF2_VERSION >= 4.
    (out_debug_info): Use difference between start and end as data
    value for DW_AT_high_pc when DWARF2_VERSION >= 4.
    * config/tc-ia64.h (DWARF2_LINE_VERSION): Override it.