Speculatively adjust gtest's UnitTest::AddTestPartResult() to not rely on...

Speculatively adjust gtest's UnitTest::AddTestPartResult() to not rely on volatile store to null trapping

This fallback path is used at least on PPC.
If this doesn't work on some compilers that take this path,
then this will have to be changed to either abort,
or partitioned to do different things based on the compiler.

Please refer to https://reviews.llvm.org/D105338.