feat: POSIX libc surface, native binutils (as/ld/ar run on MontaukOS)

Grow the Montauk libc enough to cross-build binutils 2.43.1 with
--host=x86_64-montauk. gas, ld, ar, nm, objcopy, objdump, readelf,
ranlib, strip and friends now link as native ET_EXEC Montauk binaries
(staged stripped in toolchain/native/, not yet shipped in the image).

New libc surface: full Linux errno and signal sets, O_* flags with
real O_EXCL/O_APPEND semantics, unlink/rmdir/dup/dup2/getpid/_exit,
kill (SYS_KILL), fcntl, fileno/fdopen, putc/getchar/rewind,
ctime/asctime, strtoll/strtoull/atoll, bsearch, mkstemp/mktemp,
realpath (lexical, drive-prefix aware), mbstowcs/mblen, chmod/fchmod/
umask/utime no-ops (VFS has no modes or settable times), full struct
stat with fake inodes, sscanf field widths (bounded conversions),
SCN*/PRI* completions, wait/waitpid over SYS_WAITPID, and new headers
sys/wait.h, sys/param.h, utime.h, wchar.h, memory.h.

fork/exec/pipe are declared but fail with ENOSYS: binutils never
spawns, and real process plumbing is the posix_spawn milestone, which
needs kernel support (exit status reporting, fd redirection).

TCC's montauk_compat.h shims (unlink, chmod, execvp, realpath, fdopen,
strtoll, strtoull) are retired in favor of the libc versions.

Co-Authored-By: Claude Fable 5 <[email protected]>

programs/include/libc/fcntl.h

@@ -7,13 +7,27 @@
 extern "C" {
 #endif
 
-#define O_RDONLY 0
-#define O_WRONLY 1
-#define O_RDWR   2
-#define O_CREAT  0x40
-#define O_TRUNC  0x200
+#define O_RDONLY   0
+#define O_WRONLY   1
+#define O_RDWR     2
+#define O_ACCMODE  3
+#define O_CREAT    0x40
+#define O_EXCL     0x80
+#define O_TRUNC    0x200
+#define O_APPEND   0x400
+#define O_NONBLOCK 0x800
+#define O_CLOEXEC  0x80000
+
+#define F_DUPFD  0
+#define F_GETFD  1
+#define F_SETFD  2
+#define F_GETFL  3
+#define F_SETFL  4
+
+#define FD_CLOEXEC 1
 
 int open(const char *path, int flags, ...);
+int fcntl(int fd, int cmd, ...);
 
 #ifdef __cplusplus
 }