linux-user: Clean up hostdep.h header guards

These headers all use QEMU_HOSTDEP_H as header guard symbol.  Reuse of
the same guard symbol in multiple headers is okay as long as they
cannot be included together.

Since we can avoid guard symbol reuse easily, do so: use guard symbol
$target_HOSTDEP_H for linux-user/host/$target/hostdep.h.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
