kernel - x86-64 - normalize the location of the pcb/md_regs and remove gd_rsp0

kernel - x86-64 - normalize the location of the pcb/md_regs and remove gd_rsp0

* Normalize the location of the pcb and related user process register
  frame for traps and system calls.

* The pcb is now 16-byte aligned at the top of the kernel thread's stack
  and md_regs is placed just below it.

* We no longer reserve 16 bytes for a 8086 emulation frame.

* Change the md_regs assignment in the trap code to an assertion that the
  value is correct.

* Remove gd_rsp0, use the value from common_tss.tss_rsp0 instead (which
  is the same).