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 <noreply@anthropic.com>

toolchain/patches/gcc-14.2.0-montauk.patch

@@ -47,3 +47,15 @@
      *-musl* )
  	#  IF there is no include fixing,
  	#  THEN create a no-op fixer and exit
+--- a/libstdc++-v3/configure
++++ b/libstdc++-v3/configure
+@@ -27340,6 +27340,9 @@
+   # on a hosted environment.
+   if test "x${with_newlib}" = "xyes"; then
+     os_include_dir="os/newlib"
++    case "${host}" in
++      *-montauk*) os_include_dir="os/generic" ;;  # Montauk libc is not newlib
++    esac
+     $as_echo "#define HAVE_HYPOT 1" >>confdefs.h
+ 
+