Arm backend: Fix output_folder not created issue in scripts (#9269)

In files build_executorch_runner.sh and examples/arm/run.sh, realpath
command is called on output_folder before checking whether the directory
already exists. This will trigger the error like "realpath:
${output_folder}: No such file or directory."

Fix it by moving mkdir forward.

Signed-off-by: Yufeng Shi <yufeng.shi@arm.com>