Synchronization packets for privilege change

[AoteJin]

Based on the main text of e-trace spec, the synchronization packets are not yielded for privilege change (e.g. xret):

But the Figure 2. Instruction delta trace algorithm implies that synchronization packets are generated for privilege change.
It would be better to explicitly include privilege change as a condition of synchronization in main text.

Besides, the the privilege field in format 3 subformat 0 says:

Regarding the case with instruction xret, ecall, ebreak, it is better to indicate the the privilege field holds the new privilege after execution of the instruction.

[IainCRobertson]

@AoteJin please check that #159 addresses your concern