Change Big Endian backup implementations to use signed indexes

load64() counted down from 7..0, but the decrement turned 0 into 255.
This means the loop would never terminate on Big Endian systems.

Just use signed char integers since we're only dealing with values from 0..7 anyway.

Closes https://bugs.php.net/bug.php?id=73282

ext/hash/hash_sha3.c

@@ -38,7 +38,7 @@ static inline unsigned char idx(unsigned char x, unsigned char y) {
 
 #ifdef WORDS_BIGENDIAN
 static inline uint64_t load64(const unsigned char* x) {
-	unsigned char i;
+	char i;
 	uint64_t ret = 0;
 	for (i = 7; i >= 0; --i) {
 		ret <<= 8;
@@ -47,14 +47,14 @@ static inline uint64_t load64(const unsigned char* x) {
 	return ret;
 }
 static inline void store64(unsigned char* x, uint64_t val) {
-	unsigned char i;
+	char i;
 	for (i = 0; i < 8; ++i) {
 		x[i] = val & 0xFF;
 		val >>= 8;
 	}
 }
 static inline void xor64(unsigned char* x, uint64_t val) {
-	unsigned char i;
+	char i;
 	for (i = 0; i < 8; ++i) {
 		x[i] ^= val & 0xFF;
 		val >>= 8;