Clean up linker flag definitions.

Clean up linker flag definitions.

 1. GNU ld does not understand -m{32,64}. It must be cooked for it by
    the GCC driver program.
 2. Where GNU ld is directly called we must use -melf_{i386,x86_64}.
 3. We cannot avoid calling GNU ld directly in some cases (e.g., when
    specifying GNU-specific linker scripts) as on some host
    architectures the GCC driver is configured to call the host
    linker.
 4. We cannot add -melf_{i386,x86_64} to LDFLAGS as the option is
    not recognised by GCC.

Hence we define new LDFLAGS_DIRECT, to be added to the command line
only when invoking GNU ld directly.

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