x86/asm: Further improve segment.h readability

 - extend/clarify explanations where necessary

 - move comments from macro values to before the macro, to
   make them more consistent, and to reduce preprocessor overhead

 - sort GDT index and selector values likewise by number

 - use consistent, modern kernel coding style across the file

 - capitalize consistently

 - use consistent vertical spacing

 - remove the unused get_limit() method (noticed by Andy Lutomirski)

No change in code (verified with objdump -d):

 64-bit defconfig+kvmconfig:

   815a129bc1f80de6445c1d8ca5b97cad  vmlinux.o.before.asm
   815a129bc1f80de6445c1d8ca5b97cad  vmlinux.o.after.asm

 32-bit defconfig+kvmconfig:

   e659ef045159ddf41a0771b33a34aae5  vmlinux.o.before.asm
   e659ef045159ddf41a0771b33a34aae5  vmlinux.o.after.asm
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>