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\+".

Cherry-picked from qtbase/faea8d1056e4b034404febd0ef44a00e7784018d.

Change-Id: I462e070992ff214a7261d45c208f2e5f3e962e8f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>