Fix the linker script parser so that it will recognise the PT_GNU_RELRO segment type...

Fix the linker script parser so that it will recognise the PT_GNU_RELRO segment type, and the linker itself so that it will gracefully handle being unable to assign any sections to such a segment.

	PR 28452
bfd	* elf.c (assign_file_positions_for_non_load_sections): Replace
	assertion with a warning message.

ld	* ldgram.y: Add support for PT_GNU_RELRO and PT_GNU_PROPERTY.
	* ldgram.c: Regenerate.