feat: build userspace programs with the x86_64-montauk toolchain

Switch all program and library Makefiles from host g++ (or the
bare-metal x86_64-elf compiler) to x86_64-montauk-g++/gcc and drop the
flags the target now owns: -nostdinc + kernel freestanding -isystem
paths, -m64/-march, -fno-PIC, -mno-red-zone, -mcmodel=small, -static,
-Wl,-m,elf_x86_64, -z max-page-size, --build-id=none. Deliberate
policy flags stay (-ffreestanding, per-app SSE, -nostdlib + link.ld).

Program builds now require the montauk toolchain and fail with a
pointer to build-montauk-toolchain.sh; lib/ and libs/ keep the host
fallback since the toolchain script bootstraps libc through it.
Userspace is also now pinned to the cross GCC instead of drifting
with the distro compiler.

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

programs/src/lua/Makefile

@@ -1,13 +1,13 @@
 MAKEFLAGS += -rR
 .SUFFIXES:
 
-TOOLCHAIN_PREFIX := $(shell cd ../../.. && pwd)/toolchain/local/bin/x86_64-elf-
-ifneq ($(wildcard $(TOOLCHAIN_PREFIX)gcc),)
+TOOLCHAIN_PREFIX := $(shell cd ../../.. && pwd)/toolchain/local/bin/x86_64-montauk-
+ifeq ($(wildcard $(TOOLCHAIN_PREFIX)gcc),)
+    CC = $(error x86_64-montauk toolchain not found. Run ./toolchain/build-montauk-toolchain.sh first)
+    AR = $(error x86_64-montauk toolchain not found. Run ./toolchain/build-montauk-toolchain.sh first)
+else
     CC := $(TOOLCHAIN_PREFIX)gcc
     AR := $(TOOLCHAIN_PREFIX)ar
-else
-    CC := gcc
-    AR := ar
 endif
 
 SRCDIR   := src
@@ -26,19 +26,13 @@ CFLAGS := \
     -Wno-sign-compare \
     -Wno-misleading-indentation \
     -Wno-implicit-fallthrough \
-    -nostdinc \
     -ffreestanding \
     -fno-stack-protector \
     -fno-stack-check \
-    -fno-PIC \
     -ffunction-sections \
     -fdata-sections \
-    -m64 \
-    -march=x86-64 \
     -msse \
     -msse2 \
-    -mno-red-zone \
-    -mcmodel=small \
     -D__MONTAUKOS__=1 \
     -DLUA_COMPAT_5_3 \
     -I$(SRCDIR) \
@@ -47,11 +41,7 @@ CFLAGS := \
 
 LDFLAGS := \
     -nostdlib \
-    -static \
-    -Wl,--build-id=none \
     -Wl,--gc-sections \
-    -Wl,-m,elf_x86_64 \
-    -z max-page-size=0x1000 \
     -T ../../link.ld
 
 CORE_SRCS := \