Move common mutex implementations to a MI place.

Archs not yet converted can to the jump by defining __USE_MI_MUTEX.

ok visa@

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).

replace the asm mutexes with a c implementation.

there's no real functional advantage to this, except that it will
make it easier to add deadlock detection to the code.

this is modelled on the c mutex implementation thats on alpha,
mips64, and hppa.

ok mpi@ kettenis@

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 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.

tested by mpi@, landry@
ok mpi@, dlg@

Define MUTEX_OLDIPL where they were missing, will be used (reasonably) shortly.

Move powerpc to __HAVE_MUTEX. With help from drahn@. Tested by nick@, xsa@,
deraadt@.

"reads right" deraadt@

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).

replace the asm mutexes with a c implementation.

there's no real functional advantage to this, except that it will
make it easier to add deadlock detection to the code.

this is modelled on the c mutex implementation thats on alpha,
mips64, and hppa.

ok mpi@ kettenis@

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 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.

tested by mpi@, landry@
ok mpi@, dlg@

Define MUTEX_OLDIPL where they were missing, will be used (reasonably) shortly.

Move powerpc to __HAVE_MUTEX. With help from drahn@. Tested by nick@, xsa@,
deraadt@.

"reads right" deraadt@