Have readelf warn users if it is asked to decode a LLVM bitcode file or a golang...

Have readelf warn users if it is asked to decode a LLVM bitcode file or a golang object file.

	* readelf.c (check_magic_number): New function.  Checks the magic
	bytes at the start of a file.  If they are not the ELF format
	magic values, then attempts to generate a helpful error message.
	(process_file_header): Call check_magic_number.