If a trap is encountered upon executing iretq from within doreti() the
hardware will ensure the stack pointer is aligned to a 16-byte
boundary before saving the fault state on the stack.

In the PTI case, handle this potential alignment adjustment by copying
both frames independently while unwinding the stack in between.

Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D15183