Add a hook to enable load-time inspection of program headers

This hook can be used to perform additional compatibility checks
between shared libraries by inspecting custom program header
information.

	* elf/dl-machine-reject-phdr.h: New file.
	* elf/dl-load.c: #include that.
	(open_verify): Call elf_machine_reject_phdr_p and ignore the file
	if that returned true.