set all three of the kernel, supervisor, and user-mode 64-bit mode
flags. While FreeBSD does not currently use the supervisor ring (and
hence this is effectively a NOP on most systems), doing this avoids
triggering an exception on 64-bit MIPS CPUs that don't support 32-bit
compatibility mode, and therefore don't allow clearing the SX bit.
Reviewed by: gonzo
MFC after: 3 days
Sponsored by: DARPA, SRI International
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