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>

programs/include/libc/math.h

@@ -11,8 +11,47 @@ extern "C" {
 #define INFINITY  __builtin_inff()
 #define NAN       __builtin_nanf("")
 
+/* C99 floating-point classification. */
+#define FP_NAN       0
+#define FP_INFINITE  1
+#define FP_ZERO      2
+#define FP_SUBNORMAL 3
+#define FP_NORMAL    4
+
+#define fpclassify(x) \
+    __builtin_fpclassify(FP_NAN, FP_INFINITE, FP_NORMAL, FP_SUBNORMAL, \
+                         FP_ZERO, x)
+#define isnan(x)    __builtin_isnan(x)
+#define isinf(x)    __builtin_isinf(x)
+#define isfinite(x) __builtin_isfinite(x)
+#define isnormal(x) __builtin_isnormal(x)
+#define signbit(x)  __builtin_signbit(x)
+
 double fabs(double x);
 double frexp(double x, int *exp);
+
+/* C89 modf and the C99 float variants assumed by hosted libstdc++
+   (--with-newlib crossconfig). The float forms wrap the double
+   implementations. */
+double modf(double x, double *iptr);
+float  modff(float x, float *iptr);
+double hypot(double x, double y);
+float  hypotf(float x, float y);
+float  acosf(float x);
+float  asinf(float x);
+float  atanf(float x);
+float  atan2f(float y, float x);
+float  coshf(float x);
+float  expf(float x);
+float  fmodf(float x, float y);
+float  frexpf(float x, int *exp);
+float  ldexpf(float x, int exp);
+float  logf(float x);
+float  log10f(float x);
+float  powf(float x, float y);
+float  sinhf(float x);
+float  tanf(float x);
+float  tanhf(float x);
 double ldexp(double x, int n);
 long double ldexpl(long double x, int n);
 double floor(double x);