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

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

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

Depends on D154117

Reviewed By: mehdi_amini, krzysz00

Differential Revision: https://reviews.llvm.org/D154129