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/signal.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <sys/types.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -10,7 +12,26 @@ extern "C" {
 typedef int sig_atomic_t;
 typedef void (*sighandler_t)(int);
 
-#define SIGINT 2
+/* Linux signal numbering. Only SIGINT is ever delivered today; the
+   rest exist so hosted code can name them. */
+#define SIGHUP  1
+#define SIGINT  2
+#define SIGQUIT 3
+#define SIGILL  4
+#define SIGTRAP 5
+#define SIGABRT 6
+#define SIGBUS  7
+#define SIGFPE  8
+#define SIGKILL 9
+#define SIGUSR1 10
+#define SIGSEGV 11
+#define SIGUSR2 12
+#define SIGPIPE 13
+#define SIGALRM 14
+#define SIGTERM 15
+#define SIGCHLD 17
+#define SIGCONT 18
+#define SIGSTOP 19
 
 #define SIG_DFL ((sighandler_t)0)
 #define SIG_IGN ((sighandler_t)1)
@@ -18,6 +39,7 @@ typedef void (*sighandler_t)(int);
 
 sighandler_t signal(int sig, sighandler_t handler);
 int          raise(int sig);
+int          kill(pid_t pid, int sig);
 
 #ifdef __cplusplus
 }