Adapt to custom-page-sizes

cf. llvm/llvm-project#128942

Author: yamt

libc-bottom-half/headers/public/__macro_PAGESIZE.h

@@ -2,15 +2,23 @@
 #define __wasilibc___macro_PAGESIZE_h
 
 /*
- * The page size in WebAssembly is fixed at 64 KiB. If this ever changes,
- * it's expected that applications will need to opt in, so we can change
- * this.
+ * Without custom-page-sizes proposal, the page size in WebAssembly
+ * is fixed at 64 KiB.
+ *
+ * The LLVM versions with a support of custom-page-sizes proposal
+ * provides __wasm_first_page_end global to allow page-size-agnostic
+ * objects.
  *
  * If this ever needs to be a value outside the range of an `int`, the
  * `getpagesize` function which returns this value will need special
  * consideration. POSIX has deprecated `getpagesize` in favor of
  * `sysconf(_SC_PAGESIZE)` which does not have this problem.
  */
+#if __clang_major__ >= 22
+extern char __wasm_first_page_end;
+#define PAGESIZE ((unsigned long)&__wasm_first_page_end)
+#else
 #define PAGESIZE (0x10000)
+#endif
 
 #endif