x86emul: fix test harness 32-bit "clean" target handling

x86emul: fix test harness 32-bit "clean" target handling

When preparing what is now 52c37f7ab9 ("x86emul: also allow running the
32-bit harness on a 64-bit distro") I first wrongly used XEN_TARGET_ARCH
instead of XEN_COMPILE_ARCH. When realizing the mistake I forgot to also
switch around the use in the expression controlling the rule
dependencies, causing "make distclean" to fail on 64-bit distros.

Reported-by: Paul Durrant <Paul.Durrant@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Paul Durrant <Paul.Durrant@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>