gh-127146: Report uid in Emscripten + node as native uid (#136509)

Corrects the handling of getuid on emscripten, which was consistently reporting as 0.

Python/emscripten_syscalls.c

@@ -0,0 +1,19 @@
+#include "emscripten.h"
+
+// If we're running in node, report the UID of the user in the native system as
+// the UID of the user. Since the nodefs will report the uid correctly, if we
+// don't make getuid report it correctly too we'll see some permission errors.
+// Normally __syscall_getuid32 is a stub that always returns 0 but it is
+// defined with weak linkage so we can override it.
+EM_JS(int, __syscall_getuid32_js, (void), {
+    // If we're in node and we can, report the native uid
+    if (typeof process !== "undefined" && typeof process.getuid === "function") {
+        return process.getuid();
+    }
+    // Fall back to the stub case of returning 0.
+    return 0;
+})
+
+int __syscall_getuid32(void) {
+    return __syscall_getuid32_js();
+}