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/unistd.h

@@ -4,11 +4,16 @@
 #pragma once
 
 #include <stddef.h>
+#include <sys/types.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+#define STDIN_FILENO  0
+#define STDOUT_FILENO 1
+#define STDERR_FILENO 2
+
 #define F_OK 0
 #define X_OK 1
 #define W_OK 2
@@ -22,6 +27,19 @@ int    chdir(const char *path);
 char  *getcwd(char *buf, size_t size);
 int    access(const char *path, int mode);
 int    isatty(int fd);
+int    unlink(const char *path);
+int    rmdir(const char *path);
+int    dup(int fd);
+int    dup2(int oldfd, int newfd);
+pid_t  getpid(void);
+void   _exit(int status) __attribute__((noreturn));
+pid_t  fork(void);
+int    execv(const char *path, char *const argv[]);
+int    execve(const char *path, char *const argv[], char *const envp[]);
+int    execvp(const char *file, char *const argv[]);
+int    pipe(int fds[2]);
+
+extern char **environ;
 
 unsigned int sleep(unsigned int seconds);