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

module/zfs/lz4.c

@@ -208,7 +208,7 @@ lz4_decompress_zfs(void *s_start, void *d_start, size_t s_len,
  * Little Endian or Big Endian?
  * Note: overwrite the below #define if you know your architecture endianness.
  */
-#if defined(_BIG_ENDIAN)
+#if defined(_ZFS_BIG_ENDIAN)
 #define	LZ4_BIG_ENDIAN 1
 #else
 /*

module/zfs/vdev_raidz_math_powerpc_altivec.c

@@ -225,7 +225,7 @@ const raidz_impl_ops_t vdev_raidz_powerpc_altivec_impl = {
 
 
 #if defined(__powerpc__)
-#if defined(_LITTLE_ENDIAN) && _LITTLE_ENDIAN
+#if defined(_ZFS_LITTLE_ENDIAN) && _LITTLE_ENDIAN
 /* BEGIN CSTYLED */
 const uint8_t
 __attribute__((aligned(256))) gf_clmul_mod_lt[4*256][16] = {