ffi 1.15: xfail — Android cross-compile of #jni_call(void)

Adds `examples/ffi-jni-call-02-void.sx` exercising `#jni_call(void)
(env, target, "name", "sig")` inside an `inline if OS == .android`
arm, plus a new tuple in `tests/cross_compile.sh`. Host run_examples
passes (the inline-if strips the JNI body, leaving "skipped"); the
Android cross-compile FAILs because `lowerFfiIntrinsicCall` still
emits the placeholder diagnostic for any `fic.kind != .objc_call`.

Per the FFI cadence rule this is a test-add (xfail); the next
commit makes the Android cross-compile green by adding the
`.jni_msg_send` opcode and its emit_llvm expansion.

tests/cross_compile.sh

@@ -27,6 +27,7 @@ mkdir -p "$TMP_DIR"
 # host to actually run it.
 TUPLES=(
     "android|examples/ffi-objc-call-10-os-gate.sx"
+    "android|examples/ffi-jni-call-02-void.sx"
 )
 
 PASS=0

tests/expected/ffi-jni-call-02-void.exit

@@ -0,0 +1 @@
+0

tests/expected/ffi-jni-call-02-void.txt

@@ -0,0 +1 @@
+skipped (not android)