Convert sparc64 to MI mutex.

ok dlg@

Define and use IPL_MPFLOOR in our common mutex implementation.

ok kettenis@, visa@

Unify <machine/mutex.h> a bit further.

`mtx_owner' becomes the first field of 'struct mutex' on i386/amd64/arm64.

ok visa@

Hook up mutex(9) to witness(4).

It's been a quarter century: we can assume volatile is present with that name.

ok dlg@ mpi@ deraadt@

To prevent lock ordering problems with the kernel lock, we need to make sure
we block all interrupts that can grab the kernel lock. The simplest way to
achieve this is to make sure mutexes always ra

To prevent lock ordering problems with the kernel lock, we need to make sure
we block all interrupts that can grab the kernel lock. The simplest way to
achieve this is to make sure mutexes always raise the ipl to the highest
level that has interrupts that grab the kernel lock. This will allow us
to have "mpsafe" interrupt handlers at lower priority levels.

No change for non-MULTIPROCESSOR kernels.

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Real mutexes for sparc64. Some comments from henric@ and claudio@.
Tested by fkr@, claudio@, nick@.

Simple single-processor mutex implementation, simpler than the MI code by
use of MD spl code bowels. No functional change.