cross-build: provide exterr.h and sys/exterrvar.h for non-FreeBSD hosts

with enough glue to make libc/gen/err.c compilable.

Reported by:	jhb
Reviewed by:	jhb, jrtc27
Sponsored by:	The FreeBSD Foundation
Differential revision:	https://reviews.freebsd.org/D51217

tools/build/cross-build/include/common/exterr.h

@@ -0,0 +1,14 @@
+#ifndef _EXTERR_H_
+#define _EXTERR_H_
+
+#include <sys/types.h>
+
+static inline int
+uexterr_gettext(char *buf, size_t bufsz)
+{
+	if (bufsz > 0)
+		buf[0] = '\0';
+	return (0);
+}
+
+#endif

tools/build/cross-build/include/common/sys/exterrvar.h

@@ -0,0 +1,6 @@
+#ifndef _SYS_EXTERRVAR_H_
+#define _SYS_EXTERRVAR_H_
+
+#define	UEXTERROR_MAXLEN	256
+
+#endif