[OpenMP] Change initialization of __kmp_global

There's no need to initialize variables with static storage duration
because they're implicitly initialized to zero. See
https://en.cppreference.com/w/c/language/initialization#Implicit_initialization

I think that's already relied upon because the supplied 0 only sets
'kmp_time_global_t g_time;' in 'struct kmp_base_global'. The other fields
are not set in the code, but implicitly initialized by the compiler.

Differential Revision: https://reviews.llvm.org/D66292

llvm-svn: 370943