Fix makefiles for hvmloader and vmxassist for FC5 on x86_64

Fix makefiles for hvmloader and vmxassist for FC5 on x86_64
systems.

In FC5, gcc has issues with the command lines used to build
hvmloader and vmxassist.  Basically, the compiler doesn't
like the combination of -m64 and -m32.  As these are 32 bit
apps, and intended to be, take a modified approach and simply
define XEN_TARGET_ARCH for 32 bits.  This cleans up the
command line and builds the images correctly.

From: Ben Thomas <ben@virtualiron.com>

Signed-off-by: Keir Fraser <keir@xensource.com>