Strip .debug_gnu_pubnames and .debug_gnu_pubtypes when building .gdb_index.

When not stripping those sections, there was a bug where gold was incorrectly
adjusting section-relative offsets by the offset of the input section within
the output section. That adjustment was both unnecessary and wrong, causing
gold to miss a number of debug entries that should have been added to
.gdb_index.  (With stripping, the adjustment was always 0, so the bug in
dwarf_reader.cc would have been hidden by the change to layout.cc.)

gold/
	* dwarf_reader.h (Dwarf_pubnames_table): Remove output_section_offset_.
	* dwarf_reader.cc (Dwarf_pubnames_table::read_section): Likewise.
	(Dwarf_pubnames_table::read_header): Likewise.
	* layout.cc (gdb_fast_lookup_sections): Add .debug_gnu_pubnames and
	.debug_gnu_pubtypes.
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