swab: Correctly treat the data as misaligned

The __aligned attribute in the previous version applied to the location
of the pointers, not the data the pointers pointed to.  While this
could be fixed by applying the attribute to a local typedef of uint16_t,
just using memcpy() for the unaligned access is simpler and ISO C.

This fixes the build on CHERI architectures which do not support
misaligned pointers and were thus failing with:

lib/libc/string/swab.c:12:18: error: alignment (1) of 'const uint16_t *' (aka 'const unsigned short *') is less than the required capability alignment (16) [-Werror,-Wcheri-capability-misuse]
   12 |         const uint16_t *f __aligned(1) = from;
      |

Co-authored by:	Jessica Clarke <jrtc27@FreeBSD.org>
Fixes:		02ebbc781f ("swab: Fix implementation to support overlapping copies")
Sponsored by:	AFRL, DARPA

Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D54399

lib/libc/string/swab.c

@@ -3,14 +3,16 @@
  * Copyright (c) 2024 rilysh <nightquick@proton.me>
  */
 
+#include <string.h>
 #include <unistd.h>
 #include <sys/endian.h>
 
 void
 swab(const void * __restrict from, void * __restrict to, ssize_t len)
 {
-	const uint16_t *f __aligned(1) = from;
-	uint16_t *t __aligned(1) = to;
+	const char *f = from;
+	char *t = to;
+	uint16_t tmp;
 
 	/*
 	 * POSIX says overlapping copy behavior is undefined, however many
@@ -19,7 +21,12 @@ swab(const void * __restrict from, void * __restrict to, ssize_t len)
 	 * and swapping them before writing them back accomplishes this.
 	 */
 	while (len > 1) {
-		*t++ = bswap16(*f++);
+		memcpy(&tmp, f, 2);
+		tmp = bswap16(tmp);
+		memcpy(t, &tmp, 2);
+
+		f += 2;
+		t += 2;
 		len -= 2;
 	}
 }