Adds a new label DR_NOTE_REG_BARRIER at each rseq endpoint which is read by drreg and treated as a barrier where all app values are brought back into their registers. This ensures the native rseq copy has the right input values for cases such as drbbdup where with multiple app block copies registers might not be restored until after the final block instr.
Augments the tool.drcacheoff.rseq test to load from enough registers (on x86 at least) to force drreg to use one of them, reproducing the crash. With the fix, the crash disappears.
Issue: #5658