amd64: do not handle fs/gs bases conditionally on the selector

For machines with RDFSBASE support, we only saved and restored fs and gs
base registers when corresponding segment register contained the
predefined value. This breaks some valid uses of the LDT together with
WR{F,G}SBASE.

Unconditionally save bases, and restore them on return to usermode. The
PCB_FULL_IRET optimization is still in place.

Reviewed by: markj, olce
Reported and tested: Alex S <iwtcex@gmail.com>
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D50414