readelf: print .debug_cu_index and .debug_tu_index sections

readelf: print .debug_cu_index and .debug_tu_index sections

DWARF Package files (.dwp) might contain a .debug_cu_index and/or
.debug_tu_index section. Print the contents of these sections. Handles
version 2 (DWARF4 + GNU Debugfission extension), version 5
(standardized) and version 6 (prelimenary, adds 64bit offsets).  Use
the existing dwp4 and dwp5 testfiles.

	* src/readelf.c (options): Add cu_index to debug-dump docs.
	(section_e): Add section_cu_index.
	(section_all): Likewise.
	(parse_opt): handle "cu_index" arg for debug-dump.
	(dwarf_section_short_string): New static function.
	(print_cu_index_section): Likewise.
	(print_debug): Move up implicit_info and explicit_info. Only
	search for skeleton file if .debug_info is needed. Handle both
	.debug_cu_index and .debug_tu_index through
	print_cu_index_section.
	* tests/run-readelf-cu-index.sh: New test.
	* tests/Makefile.am (TESTS): Add run-readelf-cu-index.sh.
	(EXTRA_DIST): Likewise.

Signed-off-by: Mark Wielaard <mark@klomp.org>