[OpenMP] Add omp_get_device_num() and update several other device API functions

Add omp_get_device_num() function for 5.0 which returns the number of the device
the current thread is running on. Also, did some cleanup and updating of device
API functions to make them into weak functions that should be replaced with
libomptarget functions when libomptarget is present.

Patch by Terry Wilmarth

Differential Revision: https://reviews.llvm.org/D54342

llvm-svn: 347799