[CMAKE] Prepend local compiler installation to PATH instead of appending

Also, use the proper pathlist separator on Windows

cmake/arm-none-eabi-checks.cmake

@@ -88,7 +88,12 @@ endfunction()
 function(arm_none_eabi_gcc_add_path)
     arm_none_eabi_gcc_distname(dist_name)
     set(gcc_path "${TOOLS_DIR}/${dist_name}/bin")
-    set(ENV{PATH} "$ENV{PATH}:${gcc_path}")
+    if(CMAKE_HOST_SYSTEM MATCHES ".*Windows.*")
+        set(sep "\\;")
+    else()
+        set(sep ":")
+    endif()
+    set(ENV{PATH} "${gcc_path}${sep}$ENV{PATH}")
 endfunction()
 
 function(arm_none_eabi_gcc_check)