both 64bit and 32bit binaries, not for 64bit only.
The set of the flag is not neccessary there, because the only current
user of the cpu_set_user_tls() is create_thread(), which calls
cpu_set_upcall() before and cpu_set_upcall() itself sets PCB_FULL_IRET.
Change the function for consistency and preserve existing KPI for now.
MFC after: 1 week