LibC: Add endian.h

This is part of POSIX issue 2024 and some software depends on it

userspace/libraries/LibC/include/endian.h

@@ -0,0 +1,41 @@
+#ifndef _ENDIAN_H
+#define _ENDIAN_H 1
+
+// https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/endian.h.html
+
+#include <sys/cdefs.h>
+
+#include <stdint.h>
+
+#define LITTLE_ENDIAN 0
+#define BIG_ENDIAN 1
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define BYTE_ORDER LITTLE_ENDIAN
+#endif
+
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define BYTE_ORDER BIG_ENDIAN
+#endif
+
+__BEGIN_DECLS
+
+uint16_t be16toh(uint16_t);
+uint32_t be32toh(uint32_t);
+uint64_t be64toh(uint64_t);
+
+uint16_t htobe16(uint16_t);
+uint32_t htobe32(uint32_t);
+uint64_t htobe64(uint64_t);
+
+uint16_t htole16(uint16_t);
+uint32_t htole32(uint32_t);
+uint64_t htole64(uint64_t);
+
+uint16_t le16toh(uint16_t);
+uint32_t le32toh(uint32_t);
+uint64_t le64toh(uint64_t);
+
+__END_DECLS
+
+#endif