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/files/cxx-test.cpp

@@ -0,0 +1,35 @@
+#include <cstdio>
+#include <string>
+#include <vector>
+#include <stdexcept>
+#include <memory>
+#include <algorithm>
+
+static int ctor_ran = 0;
+struct Global { Global() { ctor_ran = 42; } };
+static Global g;
+
+int main() {
+    int score = 0;
+    if (ctor_ran == 42) { puts("global ctor: OK"); score++; }
+
+    std::string s = "montauk";
+    s += "-cxx";
+    if (s == "montauk-cxx" && s.size() == 11) { puts("std::string: OK"); score++; }
+
+    std::vector<int> v{5, 3, 1, 4, 2};
+    std::sort(v.begin(), v.end());
+    if (v.front() == 1 && v.back() == 5) { puts("std::vector+sort: OK"); score++; }
+
+    auto p = std::make_unique<std::string>("heap");
+    if (*p == "heap") { puts("unique_ptr/new: OK"); score++; }
+
+    try {
+        throw std::runtime_error("caught!");
+    } catch (const std::exception& e) {
+        if (std::string(e.what()) == "caught!") { puts("exceptions: OK"); score++; }
+    }
+
+    printf("%d/5 passed\n", score);
+    return (score == 5) ? 0 : 1;
+}