[OpenMP][Libomptarget] Remove possible harmful copy constructor call for RTLsTy

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.

By matching this function call with the other one, we fix this bug.

Differential Revision: https://reviews.llvm.org/D95769