gdb_dwarf_index: Use a sed call that's more POSIX-compliant.

sed versions other than the GNU one often default to being POSIX-compliant,
in which case "+" (with and without escaping) is always an ordinary
character.

Achieve the same functionality in a way that works with both GNU and BSD
seds by using "xx*" insted of "x\+".

Change-Id: I1d2576a8c0e17b31f01a44d9632c57991e53780d
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>