Replace ASSERTV macro with compiler annotation

Remove the ASSERTV macro and handle suppressing unused 
compiler warnings for variables only in ASSERTs using the 
__attribute__((unused)) compiler annotation.  The annotation
is understood by both gcc and clang.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9671

lib/libspl/include/assert.h

@@ -145,7 +145,6 @@ do {									\
 #define	ASSERT0(x)		((void)0)
 #define	ASSERT(x)		((void)0)
 #define	assert(x)		((void)0)
-#define	ASSERTV(x)
 #define	IMPLY(A, B)		((void)0)
 #define	EQUIV(A, B)		((void)0)
 #else
@@ -156,7 +155,6 @@ do {									\
 #define	ASSERT0		VERIFY0
 #define	ASSERT		VERIFY
 #define	assert		VERIFY
-#define	ASSERTV(x)		x
 #define	IMPLY(A, B) \
 	((void)(((!(A)) || (B)) || \
 	    libspl_assert("(" #A ") implies (" #B ")", \

lib/libspl/include/sys/debug.h

@@ -33,4 +33,8 @@
 #define	__printflike(x, y) __attribute__((__format__(__printf__, x, y)))
 #endif
 
+#ifndef __maybe_unused
+#define	__maybe_unused __attribute__((unused))
+#endif
+
 #endif