Fix compilation error with -DLIBCXXABI_ENABLE_THREADS=OFF

The error is:

libcxxabi/src/cxa_guard_impl.h: In instantiation of ‘__cxxabiv1::{anonymous}::LibcppMutex __cxxabiv1::{anonymous}::GlobalStatic<__cxxabiv1::{anonymous}::LibcppMutex>::instance’:
libcxxabi/src/cxa_guard_impl.h:529:62:   required from here
libcxxabi/src/cxa_guard_impl.h:510:23: error: ‘__cxxabiv1::{anonymous}::LibcppMutex __cxxabiv1::{anonymous}::GlobalStatic<__cxxabiv1::{anonymous}::LibcppMutex>::instance’ has incomplete type
 _LIBCPP_SAFE_STATIC T GlobalStatic<T>::instance = {};
                       ^

llvm-svn: 359175