[libomptarget] Fix a race condition in checkDeviceAndCtors

When multiple threads invoke checkDeviceAndCtors, both of them may read true
from the shared variable Device.HasPendingGlobals, and then invoke initLibrary
redundantly. Therefore only protecting the access to Device.HasPendingGlobals
is not sufficient to guarantee that initLibrary is invoked just once.

To fix this race condition, we move the invocation of initLibrary into the
critical section, and remove the same lock inside initLibrary.

Differential Revision: https://reviews.llvm.org/D136952