Embedded chacha: Distinguish via dedicated macro

Set embedding expectations via CHACHA_EMBED macro rather than _KERNEL
definition.

No functional change.

Sponsored by:	Dell EMC Isilon

lib/libc/gen/arc4random.c

@@ -41,6 +41,7 @@ __FBSDID("$FreeBSD$");
 #include "libc_private.h"
 #include "un-namespace.h"
 
+#define CHACHA_EMBED
 #define KEYSTREAM_ONLY
 #include "chacha.c"
 

sys/crypto/chacha20/chacha.h

@@ -20,10 +20,10 @@ Public domain.
 #define CHACHA_STATELEN		(CHACHA_NONCELEN+CHACHA_CTRLEN)
 #define CHACHA_BLOCKLEN		64
 
-#ifdef _KERNEL
-#define LOCAL
-#else
+#ifdef CHACHA_EMBED
 #define LOCAL static
+#else
+#define LOCAL
 #endif
 
 LOCAL void chacha_keysetup(struct chacha_ctx *x, const u_char *k, u_int kbits);