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/libloader/Makefile

@@ -2,7 +2,7 @@
 
 ARCH := x86_64
 
-TOOLCHAIN_PREFIX := $(shell cd ../../.. && pwd)/toolchain/local/bin/x86_64-elf-
+TOOLCHAIN_PREFIX := $(shell cd ../../.. && pwd)/toolchain/local/bin/x86_64-montauk-
 CXX := $(TOOLCHAIN_PREFIX)g++
 AR := $(TOOLCHAIN_PREFIX)ar
 
@@ -11,28 +11,20 @@ override CXXFLAGS := \
     -g -O2 -pipe \
     -Wall \
     -Wextra \
-    -nostdinc \
     -ffreestanding \
     -fno-stack-protector \
     -fno-stack-check \
-    -fno-PIC \
     -fno-rtti \
     -fno-exceptions \
     -ffunction-sections \
     -fdata-sections \
-    -m64 \
-    -march=x86-64 \
     -mno-80387 \
     -mno-mmx \
     -mno-sse \
     -mno-sse2 \
-    -mno-red-zone \
-    -mcmodel=small \
     -I ../../include \
     -isystem ../../include/libc \
-    -isystem ../../include/montauk \
-    -isystem ../../../kernel/freestnd-c-hdrs/x86_64/include \
-    -isystem ../../../kernel/freestnd-cxx-hdrs/x86_64/include
+    -isystem ../../include/montauk
 
 LIBDIR := ../../lib/libloader
 LEGACY_BINDIR := ../../bin