[OpenMP][libomptarget] Fix union.

Summary: To make the two parts of the union have the same size, the size of vect needs to be increased by 16 bits.

Reviewers: grokos, carlo.bertolli, caomhin, ABataev

Reviewed By: grokos, ABataev

Subscribers: fedor.sergeev, guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D44254

llvm-svn: 327040