Add a machdep.unaligned_sigbus sysctl (just like alpha does), but on
arm it is readonly (depends on architecture version).
Use machdep.unaligned_sigbus to skip the unaligned access test on arm
as well.
Skip the SIGFPE test on arm when the FPU does not provide exception handling.
Close comment.
From Henning Petersen in PR 49403.