The TSS no longer needs to be unbusied before loading the task register,
since the set_tss_desc macros set the system gate type to Available IA-32 TSS.
This obscure, uncommented legacy code can now be removed for better
readability and saves 20 bytes of code space. 

Here is a breakdown of __set_tss_desc + load_TR before and after.  The
doublefault TSS GDT setup has similar savings.

- Zachary Amsden (zach@vmware.com)

before:
c01e0556:       ba 80 66 1f c0          mov    $0xc01f6680,%edx
c01e055b:       b8 e0 8f 1b c0          mov    $0xc01b8fe0,%eax
c01e0560:       66 c7 42 00 6b 20       movw   $0x206b,0x0(%edx)
c01e0566:       66 89 42 02             mov    %ax,0x2(%edx)
c01e056a:       c1 c8 10                ror    $0x10,%eax
c01e056d:       88 42 04                mov    %al,0x4(%edx)
c01e0570:       c6 42 05 89             movb   $0x89,0x5(%edx)
                                                  ^^ not busy TSS
c01e0574:       c6 42 06 00             movb   $0x0,0x6(%edx)
c01e0578:       88 62 07                mov    %ah,0x7(%edx)
c01e057b:       c1 c8 10                ror    $0x10,%eax
c01e057e:       81 25 84 66 1f c0 ff    andl   $0xfffffdff,0xc01f6684
c01e0585:       fd ff ff
                                                      ^^ clear busy
c01e0588:       b8 80 00 00 00          mov    $0x80,%eax
c01e058d:       0f 00 d8                ltr    %ax


after:

c01e0556:       ba 80 66 1f c0          mov    $0xc01f6680,%edx
c01e055b:       b8 e0 8f 1b c0          mov    $0xc01b8fe0,%eax
c01e0560:       66 c7 42 00 6b 20       movw   $0x206b,0x0(%edx)
c01e0566:       66 89 42 02             mov    %ax,0x2(%edx)
c01e056a:       c1 c8 10                ror    $0x10,%eax
c01e056d:       88 42 04                mov    %al,0x4(%edx)
c01e0570:       c6 42 05 89             movb   $0x89,0x5(%edx)
c01e0574:       c6 42 06 00             movb   $0x0,0x6(%edx)
c01e0578:       88 62 07                mov    %ah,0x7(%edx)
c01e057b:       c1 c8 10                ror    $0x10,%eax
c01e057e:       b8 80 00 00 00          mov    $0x80,%eax
c01e0583:       0f 00 d8                ltr    %ax