compat32: provide a type and a macro for (u)int64_t handling on non-x86 arches

uint64_t is 4-byte aligned on i386, but is 8-bytes aligned on all other
32bit arches FreeBSD supports.  Provide the freebsd32_uint64_t type and
the FU64_CP() macro, which are intended to be used where 32bit ABI uses
(u)int64_t type, and do proper layout and copying for the aggregate type.

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

@@ -36,8 +36,17 @@
 #include <sys/_ffcounter.h>
 
 /*
- * i386 is the only arch with a 32-bit time_t
+ * i386 is the only arch with a 32-bit time_t.
+ * Also it is the only arch with (u)int64_t having 4-bytes alignment.
  */
+typedef struct {
+#ifdef __amd64__
+	uint32_t val[2];
+#else
+	uint64_t val;
+#endif
+} freebsd32_uint64_t;
+
 #ifdef __amd64__
 typedef	int32_t	time32_t;
 #else

sys/sys/abi_compat.h

@@ -67,6 +67,14 @@
 	TS_CP((src), (dst), it_value);		\
 } while (0)
 
+#define	FU64_CP(src, dst, fld) do {				\
+	_Static_assert(sizeof((src).fld) == sizeof(uint64_t),	\
+	    "FU64_CP src: " #src "." #fld "is not 8 bytes");	\
+	_Static_assert(sizeof((dst).fld) == sizeof(uint64_t),	\
+	    "FU64_CP dst: " #dst "." #fld "is not 8 bytes");	\
+	memcpy(&(dst).fld, &(src).fld, sizeof(uint64_t));	\
+} while (0)
+
 #define	BT_CP(src, dst, fld) do {				\
 	CP((src).fld, (dst).fld, sec);				\
 	*(uint64_t *)&(dst).fld.frac[0] = (src).fld.frac;	\