Set CMAKE_CXX26_COMPILE_FEATURES to cxx_std_26 in toolchain file (#24954)

Set CMAKE_CXX26_COMPILE_FEATURES to cxx_std_26 in toolchain file.
Without this, when setting target_compile_features(target PUBLIC
cxx_std_26), the `-std=gnu++26` doesn't get added to the compiler flags.

Author: TheGoldMonkey

cmake/Modules/Platform/Emscripten.cmake

@@ -209,6 +209,7 @@ if (EMSCRIPTEN_FORCE_COMPILERS)
         set(CMAKE_CXX23_COMPILE_FEATURES "cxx_std_23")
         set(CMAKE_CXX_COMPILE_FEATURES "${CMAKE_CXX_COMPILE_FEATURES};cxx_std_23")
         if ("${CMAKE_VERSION}" VERSION_GREATER_EQUAL "3.25") # 3.25+
+          set(CMAKE_CXX26_COMPILE_FEATURES "cxx_std_26")
           set(CMAKE_CXX_COMPILE_FEATURES "${CMAKE_CXX_COMPILE_FEATURES};cxx_std_26")
         endif()
         if ("${CMAKE_VERSION}" VERSION_GREATER_EQUAL "3.21")