Partially revert "debian/rules: Combine shared Make args"

Partially revert "debian/rules: Combine shared Make args"

This reverts part of commit af30268ed73397d24acaee83477ff555a43df5e4.

The change caused the following FTBFS on i386:

    gcc    -Wl,-z,relro -Wl,-z,now -pthread -Wl,-soname
    -Wl,libxentoolcore.so.1 -shared -Wl,--version-script=libxentoolcore.map
    -o libxentoolcore.so.1.0 handlereg.opic
    /usr/bin/ld: i386:x86-64 architecture of input file `handlereg.opic' is
    incompatible with i386 output
    /usr/bin/ld: handlereg.opic: file class ELFCLASS64 incompatible with
    ELFCLASS32
    /usr/bin/ld: final link failed: file in wrong format
    collect2: error: ld returned 1 exit status

"For $(make_args_xen), ${XEN_TARGET_ARCH} gets $(xen_arch_$(flavour)),
but for $(make_args_tools), ${XEN_TARGET_ARCH} gets
$(xen_arch_$(DEB_HOST_ARCH))."