refactor: move the native devkit from 0:/usr to 0:/sdk (Montauk SDK)

0:/usr was a Unix transplant in an otherwise Montauk-native layout.
The SDK now lives at 0:/sdk/{bin,include,lib} (+ ldscripts under
0:/sdk/x86_64-montauk): Montauk-flavored, and a clean unit for a
future per-component installer tickbox. Native binutils is rebuilt
with --prefix=/sdk so ld's compiled-in search paths follow; future
ports configure with --prefix=/sdk.

Also add pathconf() with _PC_* names to the libc: now that realpath
exists, libiberty's lrealpath.c compiles its pathconf fallback path
unconditionally.

Shell command resolution and tab completion updated to 0:/sdk/bin.
Boot-verified in QEMU: setup, login, desktop on the new image with
zero usr/ remnants in the ramdisk.

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

programs/src/shell/exec.cpp

@@ -84,8 +84,8 @@ int exec_external(const char* cmd, const char* args) {
     scat(path, name, sizeof(path));
     if (try_exec(path, finalArgs)) return 0;
 
-    // 5. Try 0:/usr/bin/<cmd>.elf (native dev tools: as, ld, ar, ...)
-    scopy(path, "0:/usr/bin/", sizeof(path));
+    // 5. Try 0:/sdk/bin/<cmd>.elf (Montauk SDK: as, ld, ar, ...)
+    scopy(path, "0:/sdk/bin/", sizeof(path));
     scat(path, name, sizeof(path));
     scat(path, ".elf", sizeof(path));
     if (try_exec(path, finalArgs)) return 0;