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 <[email protected]>

programs/src/screenshot/Makefile

@@ -6,11 +6,11 @@ MAKEFLAGS += -rR
 
 # ---- Toolchain ----
 
-TOOLCHAIN_PREFIX := $(shell cd ../../.. && pwd)/toolchain/local/bin/x86_64-elf-
-ifneq ($(wildcard $(TOOLCHAIN_PREFIX)gcc),)
-    CXX := $(TOOLCHAIN_PREFIX)g++
+TOOLCHAIN_PREFIX := $(shell cd ../../.. && pwd)/toolchain/local/bin/x86_64-montauk-
+ifeq ($(wildcard $(TOOLCHAIN_PREFIX)gcc),)
+    CXX = $(error x86_64-montauk toolchain not found. Run ./toolchain/build-montauk-toolchain.sh first)
 else
-    CXX := g++
+    CXX := $(TOOLCHAIN_PREFIX)g++
 endif
 
 # ---- Paths ----
@@ -34,36 +34,24 @@ CXXFLAGS := \
     -Wextra \
     -Wno-unused-parameter \
     -Wno-unused-function \
-    -nostdinc \
     -ffreestanding \
     -fno-stack-protector \
     -fno-stack-check \
-    -fno-PIC \
     -fno-rtti \
     -fno-exceptions \
     -ffunction-sections \
     -fdata-sections \
-    -m64 \
-    -march=x86-64 \
     -msse \
     -msse2 \
-    -mno-red-zone \
-    -mcmodel=small \
     -MMD -MP \
     -I $(PROG_INC) \
-    -isystem $(PROG_INC)/libc \
-    -isystem $(PROJECT_ROOT)/kernel/freestnd-c-hdrs/x86_64/include \
-    -isystem $(PROJECT_ROOT)/kernel/freestnd-cxx-hdrs/x86_64/include
+    -isystem $(PROG_INC)/libc
 
 # ---- Linker flags ----
 
 LDFLAGS := \
     -nostdlib \
-    -static \
-    -Wl,--build-id=none \
     -Wl,--gc-sections \
-    -Wl,-m,elf_x86_64 \
-    -z max-page-size=0x1000 \
     -T $(LINK_LD)
 
 # ---- Libraries ----