feat: x86_64-montauk userspace cross toolchain

Add build-montauk-toolchain.sh: builds an OS-aware cross GCC/binutils
(same versions and prefix as the bare-metal x86_64-elf toolchain) whose
x86_64-montauk target links static ET_EXEC binaries against the Montauk
libc sysroot with no special flags: 4 KiB max page size for the kernel
ELF loader, -mno-red-zone by default, crt1/crti/crtn + -lc from the
sysroot, __montauk__ defined. Target patches are checked into
toolchain/patches/, the GCC target header into toolchain/files/.

sys/types.h now includes <stdint.h> so hosted code (libgcov and
friends) sees intptr_t via the stdio include chain.

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

programs/include/libc/sys/types.h

@@ -3,6 +3,12 @@
 
 #pragma once
 
+/* Fixed-width and pointer-sized integer types. Freestanding builds get
+   this from the kernel freestanding headers; the cross toolchain from
+   the GCC-provided stdint.h. Hosted code (e.g. libgcov) expects
+   intptr_t to be visible via the stdio/stdlib include chain. */
+#include <stdint.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif