x86/AMD: work around erratum 793 for 32-bit

The original change went into a 64-bit only code section, thus leaving
the issue unfixed on 32-bit. Re-order code to address this.

This is part of CVE-2013-6885 / XSA-82.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>