x86: Simplify timer_ack magic in time_32.c

Let the compiler optimize the timer_ack magic away in the 32bit timer
interrupt and put the same code into time_64.c. It's optimized out for
CONFIG_X86_IO_APIC on 32bit and for 64bit because timer_ack is const 0
in both cases.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>