[libomptarget][nvptx] Undef, weak shared variables

[libomptarget][nvptx] Undef, weak shared variables

Shared variables on nvptx, and LDS on amdgcn, are uninitialized at
the start of kernel execution. Therefore create the variables with
undef instead of zeros, motivated in part by the amdgcn back end
rejecting LDS+initializer.

Common is zero initialized, which seems incompatible with shared. Thus
change them to weak, following the direction of
https://reviews.llvm.org/rG7b3eabdcd215

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D90248