Prefix zfs internal endian checks with _ZFS

FreeBSD defines _BIG_ENDIAN BIG_ENDIAN _LITTLE_ENDIAN
LITTLE_ENDIAN on every architecture. Trying to do
cross builds whilst hiding this from ZFS has proven
extremely cumbersome.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10621

include/os/linux/spl/sys/byteorder.h

@@ -26,6 +26,15 @@
 #define	_SPL_BYTEORDER_H
 
 #include <asm/byteorder.h>
+
+#if defined(__BIG_ENDIAN) && !defined(_ZFS_BIG_ENDIAN)
+#define	_ZFS_BIG_ENDIAN
+#endif
+
+#if defined(__LITTLE_ENDIAN) && !defined(_ZFS_LITTLE_ENDIAN)
+#define	_ZFS_LITTLE_ENDIAN
+#endif
+
 #include <sys/isa_defs.h>
 
 #define	BSWAP_8(x)	((x) & 0xff)
@@ -49,7 +58,7 @@
 #define	BE_IN32(xa) \
 	(((uint32_t)BE_IN16(xa) << 16) | BE_IN16((uint8_t *)(xa)+2))
 
-#ifdef _BIG_ENDIAN
+#ifdef _ZFS_BIG_ENDIAN
 static __inline__ uint64_t
 htonll(uint64_t n)
 {