[OpenMP] Enable omp_get_num_devices() on Windows

This patch enables omp_get_num_devices() and omp_get_initial_device() on
Windows by providing an alternative to dlsym on Windows, and proposes to
add a new libomptarget entry, __tgt_get_num_devices().

Differential Revision: https://reviews.llvm.org/D96182

GitOrigin-RevId: ffb21e7f0593fde83568ae364040296fe94cf347
4 files changed