gas: Use udata for DW_AT_high_pc when emitting DWARF4

gas: Use udata for DW_AT_high_pc when emitting DWARF4

For DWARF4 DW_AT_high_pc can be expressed as constant offset from
DW_AT_low_pc which saves a relocation. Use DW_FORM_udate (uleb128)
to keep the constant value as small as possible.

gas/ChangeLog:

       * dwarf2dbg.c (out_debug_abbrev): When DWARF2_VERSION >= 4, use
       DW_FORM_udata for DW_AT_high_pc.
       (out_debug_info): Use emit_leb128_expr for DW_AT_high_pc, when
       DWARF2_VERSION >= 4.
       * read.c (emit_leb128_exp): No longer static.
       * read.h (emit_leb128_exp): Define.