relibc: guard size_t typedef against C++ name collision

When a C++ translation unit transitively includes this header (e.g.
via <string> pulling in <bits/basic_string.h>), the unqualified
'size_t' references inside libstdc++'s internal templates resolved
to this typedef instead of std::size_t, producing
'unterminated #ifndef' template parse errors during hosted-mode C++
compilation.

Mirror the pattern already used in bits/wchar-t.h: wrap the typedef
in '#ifndef __cplusplus / #endif' so C compilation sees the typedef
and C++ falls back to the libstdc++/libc++-provided std::size_t via
the standard type machinery.

src/header/bits_size-t/cbindgen.toml

@@ -1,7 +1,26 @@
 # POSIX header spec: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html
 #
 # This type is split out to prevent importing all of stddef.h into other headers.
+#
+# C++ compatibility: in C++, `size_t` is defined by the standard library
+# in namespace `std::`. A bare `typedef unsigned long size_t;` at file
+# scope in a C++ translation unit collides with `std::size_t` whenever
+# a host stdlib header (e.g. `<string>`, `<bits/basic_string.h>`) is
+# transitively included — the libstdc++ internal code references
+# unqualified `size_t` and resolves it to the first visible one, which
+# would otherwise be this typedef. The `__cplusplus` guard makes the
+# typedef a no-op under C++ compilation; the libc++/libstdc++ headers
+# then provide their own `std::size_t` via `__SIZE_TYPE__` or the
+# standard type machinery.
 include_guard = "_RELIBC_BITS_SIZE_T_H"
+after_includes = """
+/**
+ * Unsigned integer type of the result of the sizeof operator.
+ */
+#ifndef __cplusplus
+typedef unsigned long size_t;
+#endif
+"""
 language = "C"
 no_includes = true
 cpp_compat = true