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

[kib]

Summary

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.

[olce]

Some now unused code could be suppressed.

sys/amd64/amd64/exception.S

1169–1170

Now unused label, could be removed.

1181–1182

Now unused code?

1193–1194

Now unused label and code, could be removed.

[kib]

sys/amd64/amd64/exception.S

1193–1194

Yes, and the jmp above it as well.