Calculate offsets in read_dwarf2_lineblock based on is64

Reading header length and values in external line info was incorrect at
some places as it used offsets based on dw64 that came from .debug_info.
Instead, offsets should be calculated based on is64 from .debug_line.
This issue surfaced in MIPS64 port, and it was discussed at:

https://bugs.kde.org/show_bug.cgi?id=313267#c20


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13373 a5019735-40e9-0310-863c-91ae7b9d1cf9