[OpenMP] Fix transient divide by zero bug in 32-bit code

The value returned by __kmp_now_nsec() can overflow 32-bit values causing
incorrect values to be returned. The overflow can end up causing a divide
by zero error because in __kmp_initialize_system_tick(), the value
(__kmp_now_nsec() - nsec) can end up being much larger than the numerator:
1e6 * (delay + (now - goal))
during a pathological timing where the current time calculated is much larger
than nsec. When this happens, the value of __kmp_ticks_per_msec is set to zero
which is then used as the denominator in the KMP_NOW_MSEC() macro leading to
the divide by zero error.

Differential Revision: https://reviews.llvm.org/D55300

llvm-svn: 349090