[OpenMP][deviceRTLs] Added `[[clang::loader_uninitialized]]` explicitly

`[[clang::loader_uninitialized]]` is in macro `SHARED` but it doesn't
work for array like `parallelLevel`, so the variable will be zero initialized.
There is also a similar issue for `omptarget_nvptx_device_State` which is in
global address space. Its c'tor is also generated, which was not in the past when
building the `deviceRTLs` with CUDA. In this patch, we added the attribute to
the two variables explicitly.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D95550