feat: ship native dev tools and target sysroot at 0:/usr

The devkit target in programs/GNUmakefile stages the native binutils
(as, ld, ar, nm, objcopy, objdump, readelf, ranlib, strip, size,
strings, addr2line, c++filt, elfedit as *.elf) into the image at
0:/usr/bin, plus a target-side sysroot: libc headers, montauk/ and
Api/ SDK headers and the freestanding C++ set at 0:/usr/include, and
libc.a + crt1/crti/crtn objects (with empty libm/libstdc++ stand-ins)
at 0:/usr/lib. ld's ldscripts ship under 0:/usr/x86_64-montauk.

The /usr prefix matches the binutils configure prefix, and the kernel
already resolves driveless absolute paths against the cwd drive, so
compiled-in /usr/lib search paths work natively on drive 0.

The shell resolves commands from 0:/usr/bin (after 0:/os) and tab
completion lists it. When toolchain/native/ has not been built, the
devkit step skips cleanly and the image builds as before.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

programs/src/shell/exec.cpp

@@ -84,13 +84,19 @@ int exec_external(const char* cmd, const char* args) {
     scat(path, name, sizeof(path));
     if (try_exec(path, finalArgs)) return 0;
 
-    // 5. Try 0:/games/<cmd>.elf
+    // 5. Try 0:/usr/bin/<cmd>.elf (native dev tools: as, ld, ar, ...)
+    scopy(path, "0:/usr/bin/", sizeof(path));
+    scat(path, name, sizeof(path));
+    scat(path, ".elf", sizeof(path));
+    if (try_exec(path, finalArgs)) return 0;
+
+    // 6. Try 0:/games/<cmd>.elf
     scopy(path, "0:/games/", sizeof(path));
     scat(path, name, sizeof(path));
     scat(path, ".elf", sizeof(path));
     if (try_exec(path, finalArgs)) return 0;
 
-    // 6. If on a non-zero drive, also try drive root
+    // 7. If on a non-zero drive, also try drive root
     if (current_drive != 0) {
         build_drive_path(current_drive, name, path, sizeof(path));
         if (try_exec(path, finalArgs)) return 0;