Don't implement anything in the ffs family in <machine/cpufunc.h>

in the non-_KERNEL case.  This "fixes" applications that include
this "kernel-only" header and also include <strings.h> (or get
<strings.h> via the default _BSD_VISIBLE pollution in <string.h>.
In C++ there was a fatal error: the declaration specifies C linkage
but the implementation gives C++ linkage.  In C there was only a
static/extern mismatch if the headers were included in a certain order
order, and a partially redundant declaration for all include orders;
gcc emits incomplete or wrong diagnostics for these, but only for
compiling with -Wsystem-headers and certain other warning options, so
the problem was usually not seen for C.

Ports breakage reported by:	kris

sys/amd64/include/cpufunc.h

@@ -119,6 +119,8 @@ enable_intr(void)
 	__asm __volatile("sti");
 }
 
+#ifdef _KERNEL
+
 #define	HAVE_INLINE_FFS
 
 static __inline int
@@ -162,6 +164,8 @@ flsl(long mask)
 	return (mask == 0 ? mask : (int)bsrq((u_long)mask) + 1);
 }
 
+#endif /* _KERNEL */
+
 static __inline void
 halt(void)
 {

sys/i386/include/cpufunc.h

@@ -98,6 +98,8 @@ enable_intr(void)
 	__asm __volatile("sti");
 }
 
+#ifdef _KERNEL
+
 #define	HAVE_INLINE_FFS
 
 static __inline int
@@ -120,6 +122,8 @@ fls(int mask)
 	return (mask == 0 ? mask : (int)bsrl((u_int)mask) + 1);
 }
 
+#endif /* _KERNEL */
+
 static __inline void
 halt(void)
 {