[mlir][NVVM] Adds the NVVM target attribute.

**For an explanation of these patches see D154153.**

Commit message:
This patch adds the NVVM target attribute for serializing GPU modules into
strings containing cubin.

Depends on D154113 and D154100 and D154097

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D154117