RFR (XS): 7129715: MAC: SIGBUS in nsk stress test

[Roland Westrelin]

> Thank you, Roland, for fixing this

Thanks for the review.

> Please, add a comment to your change since it is different from other platforms. We may want to add the same check on other platforms to skip unneeded additional checks. But I will let Runtime group to handle that.

Would this comment be ok?
    // We test if stub is already set (by the stack overflow code
    // above) so it is not overwritten by the code that follows. This
    // check is not required on other platforms, because on other
    // platform we check for SIGSEGV only or SIGBUS only, where here
    // we have to check for both SIGSEGV and SIGBUS.

Roland.