libc++: mark as not available for on-device build

[skip ci]

packages/libc++/build.sh

@@ -7,5 +7,8 @@ TERMUX_PKG_SKIP_SRC_EXTRACT=true
 TERMUX_PKG_ESSENTIAL=true
 
 termux_step_post_make_install() {
+	if $TERMUX_ON_DEVICE_BUILD; then
+		termux_error_exit "Package '$TERMUX_PKG_NAME' is not available for on-device builds."
+	fi
 	cp "$TERMUX_STANDALONE_TOOLCHAIN/sysroot/usr/lib/${TERMUX_HOST_PLATFORM}/libc++_shared.so" $TERMUX_PREFIX/lib
 }