Undefine lowRISC macros, move UVM report to source file

[phsauter]

This addresses #238 and #250 and replaces #241.

• Solves lowRISC prim_assert macro collisions by undefining existing lowRISC assertion macros before common_cells redefines them.
• Moves the UVM assertion reporting helper package out of assertions.svh into a dedicated source file, avoiding repeated package definitions across translation units.