Add .debug_gdb_scripts, .debug_gnu_pubnames, .debug_gnu_pubtypes,
and .debug_str_offsets to lists of sections to strip or keep
when stripping debug info.

gold/
	* layout.cc (gdb_sections): Keep .debug_gdb_scripts and
	.debug_str_offsets; strip .debug_gnu_pubnames and
	.debug_gnu_pubtypes.
	(lines_only_debug_sections): Strip all four new sections.