port(stddef shim): Windows keeps the stub; Linux/macOS use GCC builtins

The first cut routed C TUs to hoststddef.h unconditionally. That regresses
MinGW: its <stddef.h> defines `errno (*_errno())`, which collides with
PR/os.h's `u8 errno;` field. And hoststddef.h's bin/../include/stddef.h path
does not exist on Linux anyway.

Split three ways: C++ -> hoststddef.h (unchanged); C on Windows -> the empty
N64 stub (unchanged, pre-existing behaviour); C on Linux/macOS -> size_t /
ptrdiff_t / NULL / offsetof from __SIZE_TYPE__ / __PTRDIFF_TYPE__ /
__builtin_offsetof (always predefined, no header, no collision).

Verified: Windows ntsc-final builds 240/240, -level_09 boots crash-free.

port/shim/stddef.h

@@ -25,15 +25,33 @@
 #undef __need_NULL
 #undef __need_wint_t
 #include "hoststddef.h"
+#elif defined(_WIN32)
+/*
+ * C TUs on MinGW: keep the empty N64 include/stddef.h stub. MinGW's CRT
+ * headers leak size_t/ptrdiff_t transitively, and its own <stddef.h> drags in
+ * the `#define errno (*_errno())` CRT macro, which collides with PR/os.h's
+ * `u8 errno;` struct field — so routing to the host header is not an option
+ * here.
+ */
+#include "include/stddef.h"
 #else
 /*
- * C TUs: the N64 include/stddef.h is an empty stub (its active body defines
- * neither size_t nor ptrdiff_t nor NULL). MinGW leaked those in transitively
- * through its CRT headers, so the stub was harmless there; glibc/GCC defines
- * them ONLY in the compiler's own <stddef.h>, so a C TU on Linux that relies
- * on <stddef.h> (e.g. include/PR/ultratypes.h's non-N64 path) fails to
- * compile. Route C TUs to the host header too — purely additive, the active
- * N64 stub typedefs nothing that could clash.
+ * C TUs on Linux/macOS: the N64 include/stddef.h stub defines nothing, glibc
+ * only defines these in the compiler's shadowed builtin <stddef.h>, and the
+ * MinGW hoststddef trick (bin/../include/stddef.h) does not exist on Linux.
+ * Provide the standard types straight from the always-predefined GCC/Clang
+ * builtins — no header, so nothing to collide with. C11 permits the benign
+ * identical redefinition if a real <stddef.h> is ever reached another way.
  */
-#include "hoststddef.h"
+#ifndef _PORT_SHIM_STDDEF_TYPES
+#define _PORT_SHIM_STDDEF_TYPES
+typedef __SIZE_TYPE__    size_t;
+typedef __PTRDIFF_TYPE__ ptrdiff_t;
+#ifndef NULL
+#define NULL ((void *)0)
+#endif
+#ifndef offsetof
+#define offsetof(t, m) __builtin_offsetof(t, m)
+#endif
+#endif
 #endif