amd64: Cleanups to setting TLS registers for Linux binaries.

amd64: Cleanups to setting TLS registers for Linux binaries.

- Use update_pcb_bases() when updating FS or GS base addresses to
  permit use of FSBASE and GSBASE in Linux processes.  This also sets
  PCB_FULL_IRET.  linux32 was setting PCB_32BIT which should be a
  no-op (exec sets it).

- Remove write-only variables to construct unused segment descriptors
  for linux32.

Reviewed by:	kib
MFC after:	1 week
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D29026