x86/time: Adjust init-time handling of pit0_ticks

x86/time: Adjust init-time handling of pit0_ticks

There is no need for the volatile cast in the timer interrupt; the compiler
may not elide the update.  This reduces the generated assembly from a read,
local modify, write to a single add instruction.

Drop the memory barriers from timer_irq_works(), as they are not needed.
pit0_ticks is only modified by timer_interrupt() running on the same CPU, so
all that is required is a volatile reference to prevent the compiler from
eliding the second read.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>