relibc: add missing POSIX/Linux headers and constants

New headers:
- include/sys/ioccom.h: Linux UAPI ioctl encoding macros (_IO/_IOR/_IOW/_IOWR)
- include/byteswap.h: bswap_16/32/64 inline functions
- src/header/sys_statfs/: struct statfs + statfs()/fstatfs() functions
  (delegates to fstatvfs, converts to Linux struct statfs format)

New constants:
- sys_socket: MSG_NOSIGNAL, MSG_PROBE, MSG_CONFIRM, MSG_MORE, MSG_FASTOPEN
- sys_mman: MAP_GROWSDOWN, MAP_DENYWRITE, MAP_EXECUTABLE, MAP_LOCKED, MAP_NONBLOCK
- elf/cbindgen.toml: ELFMAG and SELFMAG #defines in after_includes

These headers eliminate the need for shim patches in libwayland (MSG_NOSIGNAL),
mesa (sys/ioccom.h), qtbase (sys/statfs.h, ELFMAG), pipewire/wireplumber
(byteswap.h, MAP_* flags).

include/byteswap.h

@@ -0,0 +1,28 @@
+#ifndef _BYTESWAP_H
+#define _BYTESWAP_H
+
+#include <stdint.h>
+
+static inline uint16_t bswap_16(uint16_t x) {
+    return (x >> 8) | (x << 8);
+}
+
+static inline uint32_t bswap_32(uint32_t x) {
+    return ((x & 0x000000ff) << 24) |
+           ((x & 0x0000ff00) << 8)  |
+           ((x & 0x00ff0000) >> 8)  |
+           ((x & 0xff000000) >> 24);
+}
+
+static inline uint64_t bswap_64(uint64_t x) {
+    return ((x & 0x00000000000000ffULL) << 56) |
+           ((x & 0x000000000000ff00ULL) << 40) |
+           ((x & 0x0000000000ff0000ULL) << 24) |
+           ((x & 0x00000000ff000000ULL) << 8)  |
+           ((x & 0x000000ff00000000ULL) >> 8)  |
+           ((x & 0x0000ff0000000000ULL) >> 24) |
+           ((x & 0x00ff000000000000ULL) >> 40) |
+           ((x & 0xff00000000000000ULL) >> 56);
+}
+
+#endif