libdw: Handle overflowed DW_SECT_INFO offsets in DWARF package file indexes

libdw: Handle overflowed DW_SECT_INFO offsets in DWARF package file indexes

Meta uses DWARF package files for our large, statically-linked C++
applications.  Some of our largest applications have more than 4GB in
.debug_info.dwo, but the section offsets in .debug_cu_index and
.debug_tu_index are 32 bits; see the discussion here [1].  We
implemented a workaround/extension for this in LLVM.  Implement the
equivalent in libdw.

To test this, we need files with more than 4GB in .debug_info.dwo.  I
created these artificially by editing GCC's assembly output.  They
compress down to 6KB.  I test them from run-large-elf-file.sh to take
advantage of the existing checks for large file support.

1: https://discourse.llvm.org/t/dwarf-dwp-4gb-limit/63902.

 	* libdw/dwarf_end.c (dwarf_package_index_free): New function.
	* tests/testfile-dwp-4-cu-index-overflow.bz2: New test file.
	* tests/testfile-dwp-4-cu-index-overflow.dwp.bz2: New test file.
	* tests/testfile-dwp-5-cu-index-overflow.bz2: New test file.
	* tests/testfile-dwp-5-cu-index-overflow.dwp.bz2: New test file.
	* tests/testfile-dwp-cu-index-overflow.source: New file.
	* tests/run-large-elf-file.sh: Check
	testfile-dwp-5-cu-index-overflow and
	testfile-dwp-4-cu-index-overflow.

Signed-off-by: Omar Sandoval <osandov@fb.com>