fix: real argv[0] via SYS_GETEXECPATH - GCC driver finds its backends

The GCC driver relocates its install prefix from argv[0]. crt1
hardcoded argv[0] as "prog", so make_relative_prefix cwd-joined it
and computed exec prefixes relative to the current directory
(0:/users/admin/../libexec/gcc/...), and because the computed
gcc_exec_prefix is non-NULL the standard /sdk prefixes were never
searched. cc1plus was unreachable from anywhere except (by accident
of the path arithmetic) 0:/sdk/bin. sdk-diag proved the kernel and
libc layers all worked; only the driver's self-relocation was lost.

New SYS_GETEXECPATH (151) returns the absolute path the process was
spawned from (Process::name); crt1 uses it for argv[0] with a "prog"
fallback. With a real argv[0], make_relative_prefix computes
0:/sdk/bin/../libexec/gcc/ from any cwd. Native GCC relinked against
the new crt1; sdk-diag ships in the SDK as a permanent probe; the
montauk.h TCC mirror gains the wrapper (checker enforced it).

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

programs/lib/libc/crt/crt1.c

@@ -31,8 +31,9 @@ static inline long _sys2(long nr, long a1, long a2) {
     return ret;
 }
 
-#define SYS_EXIT    0
-#define SYS_GETARGS 25
+#define SYS_EXIT        0
+#define SYS_GETARGS     25
+#define SYS_GETEXECPATH 151
 
 extern int main(int argc, char** argv);
 
@@ -71,12 +72,17 @@ static void _run_fini_array(void) {
 void _start(void) {
     /* Static: 4 KiB args + 256 argv slots would crowd a 32 KiB stack. */
     static char argbuf[4096];
+    static char pathbuf[256];
     static char* argv[256];
     int len = (int)_sys2(SYS_GETARGS, (long)argbuf, (long)sizeof(argbuf));
 
     int argc = 0;
 
-    argv[argc++] = (char*)"prog";
+    /* Real argv[0]: tools like the GCC driver relocate their install
+       prefix from it, so "prog" placeholders send them to garbage
+       paths. Fall back to "prog" on kernels without the syscall. */
+    int plen = (int)_sys2(SYS_GETEXECPATH, (long)pathbuf, (long)sizeof(pathbuf));
+    argv[argc++] = (plen > 0) ? pathbuf : (char*)"prog";
 
     if (len > 0) {
         if (len > (int)sizeof(argbuf) - 1) {