Mark the return address of the call to ast() in the generic trap
handling code so the stack trace unwinders don't start trying to
go into user-space.

Found by trying to create core dumps with a KTR_COMPILE/KTR_GEOM
kernel, which results in a stack_save() call in the ast() coredump
path - this created a panic, and then calling 'trace' in ddb resulted
in the black screen of death after printing out most of the backtrace.