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