[OpenMP][OMPT] fix reduction test for 32-bit x86

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=44733 | TEST 'libomp :: ompt/synchronization/reduction/tree_reduce.c' FAILED on 32-bit x86 ]]

For 32-bit we need at least 3 variables to avoid atomic reduction to be
choosen by runtime function `__kmp_determine_reduction_method`.
This patch adds reduction variables to the testcase.

Reviewers: mgorny, Hahnfeld

Differential Revision: https://reviews.llvm.org/D73850