cmake: fix cross-compilation problems

- unexpected pkg-config module (we should not use host binary)
- bump cmake_minimum_required to 3.5 in toolchain files

platforms/linux/gnu.toolchain.cmake

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.5)
 
 # load settings in case of "try compile"
 set(TOOLCHAIN_CONFIG_FILE "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/toolchain.config.cmake")

platforms/linux/riscv.toolchain.cmake

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.5)
 
 if(COMMAND toolchain_save_config)
   return() # prevent recursive call