libfortran: Fix execute_command_line for Windows

On Windows, 'system' is called - that fails with -1 if the command
interpreter could not be started; on POSIX systems, if the child
process could not be started by the shell, exit(127)/_exit(127) is
called/returned. On Windows, cmd.exe (and also the PowerShell) return
errorlevel 9009.

libgfortran/ChangeLog:

	* intrinsics/execute_command_line.c (execute_command_line): On
	Windows, regard system()'s return value of 9009 as EXEC_INVALIDCOMMAND.