libipt: use union to convert cpuid return values into vendor string

The current method of copying the 32-bit ebx, ecx, and edx values returned by
cpuid into a vendor string array breaks gcc's strict aliasing.  This, in turn,
breaks the release build.

Use a translation union, instead.

Change-Id: I0e31921d5c49f8122b72180665cf1772e0c6b50b
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>