FreeBSD: fix world build after 143476c

Do not redefine the fallthrough macro when building with libcpp.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes openzfs#12880

lib/libspl/include/sys/feature_tests.h

@@ -30,7 +30,7 @@
 #define	____cacheline_aligned
 #define	__NORETURN		__attribute__((__noreturn__))
 
-#if !defined(fallthrough)
+#if !defined(fallthrough) && !defined(_LIBCPP_VERSION)
 #if defined(HAVE_IMPLICIT_FALLTHROUGH)
 #define	fallthrough		__attribute__((__fallthrough__))
 #else