Promote WASM Emscripten to stable tier-3 (#678)

The builder is stable for at least 2 weeks:
https://buildbot.python.org/#/builders/1810

But I missed its recent failure since it was classified as unstable.

Author: vstinner

master/custom/builders.py

@@ -183,6 +183,9 @@
     # Android
     ("aarch64 Android", "mhsmith-android-aarch64", AndroidBuild),
     ("AMD64 Android", "mhsmith-android-x86_64", AndroidBuild),
+
+    # Emscripten
+    ("WASM Emscripten", "rkm-emscripten", EmscriptenBuild),
 ]
 
 
@@ -317,9 +320,6 @@
     ("AMD64 FreeBSD15", "opsec-fbsd15", UnixBuild),
     # FreeBSD 16 is CURRENT: development branch (at 2026-01-09)
     ("AMD64 FreeBSD16", "opsec-fbsd16", UnixBuild),
-
-    # Emscripten
-    ("WASM Emscripten", "rkm-emscripten", EmscriptenBuild),
 ]