runtime: fix stackBarrier
runtime: fix stackBarrier

On sparc64, pc is the address of the current instruction, rather than the next,
so when returning to the caller of a function with a stack barrier in place, we
must add 8 to the return address (+4 for the delay slot and +4 for the next
instruction).

4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go