x86: streamline irq_lock()/irq_unlock()

The routines _int_latency_start() and _int_latency_stop() have been
replaced by macros that evaluate to nothing when the kernel config
option INT_LATENCY_BENCHMARK is not enabled thereby giving a performance
boost to the x86 versions of irq_lock() and and irq_unlock().

Change-Id: Iabfa7bf001f5b8396e7bcf5eebd6b1aa342bac46
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>