wasi-libc: define BULK_MEMORY_THRESHOLD for the bulk_memory feature (#13787)

When the bulk_memory feature is enabled, wasi-libc will only use
it if the number of bytes is >= BULK_MEMORY_THRESHOLD

Set it to 32 as in the original wasi-libc Makefile.

src/wasi_libc.zig

@@ -277,6 +277,8 @@ fn addCCArgs(
 
         "-iwithsysroot",
         try comp.zig_lib_directory.join(arena, &[_][]const u8{ "libc", "include", triple }),
+
+        "-DBULK_MEMORY_THRESHOLD=32",
     });
 }