cpus: protect TimerState writes with a spinlock

cpus: protect TimerState writes with a spinlock

In the next patch, we will need to write cpu_ticks_offset from any
thread, even outside the BQL.  Currently, it is protected by the BQL
just because cpu_enable_ticks and cpu_disable_ticks happen to hold it,
but the critical sections are well delimited and it's easy to remove
the BQL dependency.

Add a spinlock that matches vm_clock_seqlock, and hold it when writing
to the TimerState.  This also lets us fix cpu_update_icount when 64-bit
atomics are not available.

Fields of TiemrState are reordered to avoid padding.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>