linux-user: Clean up target_signal.h header guards

These headers all use TARGET_SIGNAL_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
$target_TARGET_SIGNAL_H for linux-user/$target/target_signal.h.

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