gdb: introduce objfile text_section_offset and data_section_offset methods

gdb: introduce objfile text_section_offset and data_section_offset methods

The pattern

  objfile->section_offsets[SECT_OFF_TEXT (objfile)]

... appears very often, to get the offset of the text section of an
objfile.  I thought it would be more readable to write it as:

  objfile->text_section_offset ()

... so I added this method and used it where possible.  I also added
data_section_offset, although it is not used as much.

gdb/ChangeLog:

	* objfiles.h (ALL_OBJFILE_OSECTIONS): Move up.
	(SECT_OFF_DATA): Likewise.
	(SECT_OFF_RODATA): Likewise.
	(SECT_OFF_TEXT): Likewise.
	(SECT_OFF_BSS): Likewise.
	(struct objfile) <text_section_offset, data_section_offset>: New
	methods.
	* amd64-windows-tdep.c (amd64_windows_find_unwind_info): Use
	objfile::text_section_offset.
	* coff-pe-read.c (add_pe_forwarded_sym): Likewise.
	* coffread.c (coff_symtab_read): Likewise.
	(enter_linenos): Likewise.
	(process_coff_symbol): Likewise.
	* ctfread.c (get_objfile_text_range): Likewise.
	* dtrace-probe.c (dtrace_probe::get_relocated_address):
	Use objfile::data_section_offset.
	* dwarf2-frame.c (execute_cfa_program): Use
	objfile::text_section_offset.
	(dwarf2_frame_find_fde): Likewise.
	* dwarf2read.c (create_addrmap_from_index): Likewise.
	(create_addrmap_from_aranges): Likewise.
	(dw2_find_pc_sect_compunit_symtab): Likewise.
	(process_psymtab_comp_unit_reader): Likewise.
	(add_partial_symbol): Likewise.
	(add_partial_subprogram): Likewise.
	(process_full_comp_unit): Likewise.
	(read_file_scope): Likewise.
	(read_func_scope): Likewise.
	(read_lexical_block_scope): Likewise.
	(read_call_site_scope): Likewise.
	(dwarf2_rnglists_process): Likewise.
	(dwarf2_ranges_process): Likewise.
	(dwarf2_ranges_read): Likewise.
	(dwarf_decode_lines_1): Likewise.
	(new_symbol): Likewise.
	(dwarf2_fetch_die_loc_sect_off): Likewise.
	(dwarf2_per_cu_text_offset): Likewise.
	* hppa-bsd-tdep.c (hppabsd_find_global_pointer): Likewise.
	* hppa-tdep.c (read_unwind_info): Likewise.
	* ia64-tdep.c (ia64_find_unwind_table): Likewise.
	* psympriv.h (struct partial_symtab): Likewise.
	* psymtab.c (find_pc_sect_psymtab): Likewise.
	* solib-svr4.c (enable_break): Likewise.
	* stap-probe.c (relocate_address): Use
	objfile::data_section_offset.
	* xcoffread.c (enter_line_range): Use
	objfile::text_section_offset.
	(read_xcoff_symtab): Likewise.