amd64: Cleanups to setting TLS registers for Linux binaries.

[jhb]

Summary

• 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.

[kib]

sys/amd64/linux/linux_machdep.c

293

why not update_pcb_bases() ? This name more cleanly reflects the intent there, IMO.

[jhb]

sys/amd64/linux/linux_machdep.c

293

why not update_pcb_bases() ? This name more cleanly reflects the intent there, IMO.

Hmm, there are several other places that all use PCB_FULL_IRET already in the Linuxulator. Probably they should all switch to this?

[kib]

sys/amd64/linux/linux_machdep.c

293

I looked over them.

I think that only two uses of PCB_FULL_IRET in linux_arch_prctl() are about handling fs/gsbases, and can be changed for clarity. All uses in linux_sysvec.c do want full register file reload because they modify all registers.

[jhb]

• Use update_pcbbases() in a few more places.