feat: hosted libstdc++ for x86_64-montauk - full C++ on MontaukOS

The cross toolchain now builds real hosted C++: std::string, vector,
unique_ptr, iostreams and exceptions all work in a plain
x86_64-montauk-g++ invocation with no special flags. This is the
library that native cc1plus will link against for the GCC self-host.

crt1 now runs .preinit_array/.init_array before main and hooks
.fini_array through atexit (main returns via exit(), so destructors
run on both exit paths). Global constructors and libgcc's eh_frame
registration both ride this. GCC is reconfigured with
--enable-initfini-array (modern arrays instead of .ctors),
--with-newlib (satisfies libstdc++'s crossconfig for unknown hosts),
--disable-wchar_t and --disable-libstdcxx-pch; the montauk gcc patch
grows a hunk keeping os/generic ctype under --with-newlib (the
Montauk libc is not newlib).

libc additions harvested by the libstdc++ build: mbtowc/wctomb,
strxfrm, strtok/strtok_r, a real vfscanf/fscanf/scanf (character-
driven with widths and l/ll; also unblocks gprof later), fgetpos/
fsetpos/setbuf/fpos_t, modf plus the C99 float math variants
(acosf..tanhf, hypot/hypotf as wrappers), FP_* classification macros,
the full POSIX errno vocabulary, complete DT_* dirent types, and
mbstate_t.

The SDK ships the real libstdc++.a (was an empty stand-in), the C++
headers at 0:/sdk/include/c++, and cxx-test.elf (global ctor, string,
vector+sort, unique_ptr, throw/catch; exits 0 on 5/5). Ramdisk sits
at 1571 files, within the 2048 table.

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

toolchain/build-montauk-toolchain.sh

@@ -128,10 +128,15 @@ cp "${REPO_DIR}/programs/lib/libc/crt1.o" \
    "${REPO_DIR}/programs/lib/libc/crti.o" \
    "${REPO_DIR}/programs/lib/libc/crtn.o" "${SYSROOT}/usr/lib/"
 cp "${REPO_DIR}/programs/lib/libc/liblibc.a" "${SYSROOT}/usr/lib/libc.a"
-# Empty stand-ins: math functions live inside libc.a, and there is no
-# libstdc++ yet, but the g++ driver links -lstdc++ -lm unconditionally.
+# libm is an empty stand-in (math lives inside libc.a; the g++ driver
+# links -lm unconditionally). libstdc++ is copied from the toolchain
+# once built - the empty stand-in only bridges the first bootstrap.
 ar rcs "${SYSROOT}/usr/lib/libm.a"
-ar rcs "${SYSROOT}/usr/lib/libstdc++.a"
+if [[ -f "${PREFIX}/x86_64-montauk/lib/libstdc++.a" ]]; then
+    cp "${PREFIX}/x86_64-montauk/lib/libstdc++.a" "${SYSROOT}/usr/lib/libstdc++.a"
+else
+    ar rcs "${SYSROOT}/usr/lib/libstdc++.a"
+fi
 green "Sysroot ready."
 
 # ── Build Binutils ────────────────────────────────────────────────────────────
@@ -177,13 +182,19 @@ else
             --disable-multilib \
             --disable-gcov \
             --with-gnu-as \
-            --with-gnu-ld
+            --with-gnu-ld \
+            --with-newlib \
+            --enable-initfini-array \
+            --disable-wchar_t \
+            --disable-libstdcxx-pch
         make -j"${JOBS}" all-gcc
         make -j"${JOBS}" all-target-libgcc
         make install-gcc
         make install-target-libgcc
+        make -j"${JOBS}" all-target-libstdc++-v3
+        make install-target-libstdc++-v3
     )
-    green "GCC installed."
+    green "GCC and libstdc++ installed."
 fi
 
 # ── Done ──────────────────────────────────────────────────────────────────────