emscripten: mention ndk-sysroot in test notes

Well yes, as a byproduct of passing emscripten test suite, you can use
close-to-upstream emscripten-llvm clang to build native Android
binaries.
Starting LLVM 15, I stopped building libunwind as the build method no
longer works (most likely not supported anymore) and its unknown how
much differs from the one shipped in NDK.

You can still build native executables by installing ndk-sysroot which
includes libunwind.
emscripten-llvm will not hard depend on ndk-sysroot as the focus is
WebAssembly.

packages/emscripten/build.sh

@@ -294,7 +294,7 @@ termux_step_create_debscripts() {
 # https://github.com/emscripten-core/emscripten/issues/9098
 #
 # Steps:
-# - pkg install emscripten-tests-third-party openjdk-17
+# - pkg install emscripten-tests-third-party ndk-sysroot openjdk-17
 # - cd $PREFIX/opt/emscripten
 # - npm install --no-optional
 # - export EMTEST_SKIP_V8=1