dtrace: Add compiler barriers around updates to per-CPU flags

Otherwise there's nothing preventing reordering of memory accesses with
respect to these flags being set or cleared. Since their most common
use is to create a section in dtrace probe context where memory faults
are intercepted by dtrace_trap(), such reordering can turn dtrace errors
into fatal exceptions.

MFC after: 2 weeks
Sponsored by: Innovate UK