These headers all use TARGET_CPU_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
$target_TARGET_CPU_H for linux-user/$target/target_cpu.h.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NRichard Henderson <rth@twiddle.net>