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/sys/ioccom.h

@@ -0,0 +1,47 @@
+#ifndef _SYS_IOCCOM_H
+#define _SYS_IOCCOM_H
+
+#define _IOC_NRBITS     8
+#define _IOC_TYPEBITS   8
+#define _IOC_SIZEBITS   14
+#define _IOC_DIRBITS    2
+
+#define _IOC_NRMASK     ((1 << _IOC_NRBITS) - 1)
+#define _IOC_TYPEMASK   ((1 << _IOC_TYPEBITS) - 1)
+#define _IOC_SIZEMASK   ((1 << _IOC_SIZEBITS) - 1)
+#define _IOC_DIRMASK    ((1 << _IOC_DIRBITS) - 1)
+
+#define _IOC_NRSHIFT    0
+#define _IOC_TYPESHIFT  (_IOC_NRSHIFT + _IOC_NRBITS)
+#define _IOC_SIZESHIFT  (_IOC_TYPESHIFT + _IOC_TYPEBITS)
+#define _IOC_DIRSHIFT   (_IOC_SIZESHIFT + _IOC_SIZEBITS)
+
+#define _IOC_NONE   0U
+#define _IOC_WRITE  1U
+#define _IOC_READ   2U
+
+#define _IOC(dir, type, nr, size) \
+    (((dir)  << _IOC_DIRSHIFT)  | \
+     ((type) << _IOC_TYPESHIFT) | \
+     ((nr)   << _IOC_NRSHIFT)   | \
+     ((size) << _IOC_SIZESHIFT))
+
+#define _IOC_TYPECHECK(t) (sizeof(t))
+
+#define _IO(type, nr)            _IOC(_IOC_NONE,              (type), (nr), 0)
+#define _IOR(type, nr, argtype)  _IOC(_IOC_READ,              (type), (nr), (_IOC_TYPECHECK(argtype)))
+#define _IOW(type, nr, argtype)  _IOC(_IOC_WRITE,             (type), (nr), (_IOC_TYPECHECK(argtype)))
+#define _IOWR(type, nr, argtype) _IOC(_IOC_READ | _IOC_WRITE, (type), (nr), (_IOC_TYPECHECK(argtype)))
+
+#define _IOC_DIR(nr)   (((nr) >> _IOC_DIRSHIFT)  & _IOC_DIRMASK)
+#define _IOC_TYPE(nr)  (((nr) >> _IOC_TYPESHIFT) & _IOC_TYPEMASK)
+#define _IOC_NR(nr)    (((nr) >> _IOC_NRSHIFT)   & _IOC_NRMASK)
+#define _IOC_SIZE(nr)  (((nr) >> _IOC_SIZESHIFT) & _IOC_SIZEMASK)
+
+#define IOC_IN     (_IOC_WRITE  << _IOC_DIRSHIFT)
+#define IOC_OUT    (_IOC_READ   << _IOC_DIRSHIFT)
+#define IOC_INOUT  ((_IOC_WRITE | _IOC_READ) << _IOC_DIRSHIFT)
+#define IOCSIZE_MASK    (_IOC_SIZEMASK << _IOC_SIZESHIFT)
+#define IOCSIZE_SHIFT   (_IOC_SIZESHIFT)
+
+#endif