sys/abi_compat.h: fix UB for bintime32 handling

Do not cast and then access potentially unaligned uint64_t in the BT_CP()
macro.  Use freebsd32_uint64_t type and FU64_CP() for the frac member.

Noted by:	des
Reviewed by:	des, emaste
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D54663

sys/compat/freebsd32/freebsd32.h

@@ -70,7 +70,7 @@ struct itimerspec32 {
 
 struct bintime32 {
 	time32_t sec;
-	uint32_t frac[2];
+	freebsd32_uint64_t frac;
 };
 
 struct ffclock_estimate32 {

sys/sys/abi_compat.h

@@ -77,7 +77,7 @@
 
 #define	BT_CP(src, dst, fld) do {				\
 	CP((src).fld, (dst).fld, sec);				\
-	*(uint64_t *)&(dst).fld.frac[0] = (src).fld.frac;	\
+	FU64_CP((src).fld, (dst).fld, frac);			\
 } while (0)
 
 #endif /* !_COMPAT_H_ */