[libomptarget-nvptx] Align data sharing stack

NVPTX requires addresses of pointer locations to be 8-byte aligned
or there will be an exception during runtime.
This could happen without this patch as shown in the added test:
getId() requires 4 byte of stack and putValueInParallel() uses 16
bytes to store the addresses of the captured variables.

Differential Revision: https://reviews.llvm.org/D52655

llvm-svn: 343402