cdefs: Add __writeonly to mark write only vars

When a variable in write only and can't be removed, e.g. for API
reasons, it is useful to document this fact similar to __diagused
and __witness_used.

Add __writeonly to tell the compiler and anyone looking at the code
that this variable is expected to only be written to, and to not
raise and error.

Reviewed by:	imp, kib
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D45561

sys/sys/cdefs.h

@@ -180,6 +180,7 @@
 #define	__aligned(x)	__attribute__((__aligned__(x)))
 #define	__section(x)	__attribute__((__section__(x)))
 #endif
+#define	__writeonly	__unused
 #if __GNUC_PREREQ__(4, 3) || __has_attribute(__alloc_size__)
 #define	__alloc_size(x)	__attribute__((__alloc_size__(x)))
 #define	__alloc_size2(n, x)	__attribute__((__alloc_size__(n, x)))