Further refactor cxa_guard.cpp am: 9cbec86496 am: 14ef6a7e0e am: 98ae543535 am: 64684d7e48

Original change: https://android-review.googlesource.com/c/platform/external/libcxxabi/+/1727432

Change-Id: Ia088f12742278124d6c6792bfa911dcbb3bd9beb