x86: avoid ifdefs in desc.h, getting rid of pack_ldt and pack_tss

By Andi Kleen's suggestion, this patch removes pack_ldt() and pack_tss()
wrappers in favour of a general wrapper. It saves us an ifdef and some lines
of code, but more importantly, it's more elegant.

No functional change is made, although the object code is expected to be
different.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>