Borrowing costs for some of the euro zone’s most highly indebted southern states shot higher on Thursday, as fears of slowing economic growth wounded confidence that the European Central Bank could avert another debt crisis in the bloc. Greek government bonds were the hardest hit as a sharp sell-off gripped financial markets for a second day, with 10-year yields rising briefly over 9 percent, while Spain missed its target at a bond auction due to weak demand from investors.