From https://bugs.llvm.org/show_bug.cgi?id=48973, we know that
std::call_once(PM->RTLs.initFlag, &RTLsTy::LoadRTLs, PM->RTLs) causes compile
time problems in libstdc++v3 5.3.1. This is because there was a defect in the
standard regarding the call_once (LWG 2442). This was fixed in libstdc++ soon
thereafter, but there are likely other standard libraries where this will fail.
It seems to me that the latest libstdc++ reads PM->RTLs and knows to move
PM->RTLs, while earlier implementations try to copy it, but this seems to fix
it for me at least locally, but given how finicky this could be, it'd be great
if @sylvestre.ledru could confirm it for me. The alternative is to change the
LoadRTLs signature but this should be fine.