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/utime.h

@@ -0,0 +1,25 @@
+#ifndef _LIBC_UTIME_H
+#define _LIBC_UTIME_H
+
+#pragma once
+
+#include <sys/types.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct utimbuf {
+    time_t actime;
+    time_t modtime;
+};
+
+/* Accepted for POSIX compatibility; the Montauk VFS does not support
+   setting file times. */
+int utime(const char *path, const struct utimbuf *times);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _LIBC_UTIME_H */