x86: unify set_tss_desc

This patch unifies the set_tss_desc between i386 and x86_64,
which can now have a common implementation. After the old
functions are removed from desc_{32,64}.h, nothing important is
left, and the files can be removed.

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>