MIPS/LD: Fix crashing with a discarded dynamic relocation section

MIPS/LD: Fix crashing with a discarded dynamic relocation section

Fix a crash that occurs in `_bfd_mips_elf_finish_dynamic_sections' if a
dynamic relocation section has been created, but marked to be discarded
by an assignment to the /DISCARD/ output section in a linker script.
In that case the output section is the absolute section, which has no
ELF section data attached, so trying to set its `sh_size' parameter
causes a null pointer dereference.

This is only done as the value for the DT_RELSZ dynamic entry is being
set, so fix the problem by not creating DT_REL, DT_RELSZ or DT_RELENT
dynamic entries in the first place if the dynamic relocation section
will not be output, as with no dynamic relocation data present these
would not serve their purpose anyway.

Add a generic ELF test case to verify that no dynamic relocation data is
reported in the dynamic segment.

	bfd/
	* elfxx-mips.c (_bfd_mips_elf_size_dynamic_sections): Do not
	create DT_REL, DT_RELSZ or DT_RELENT dynamic entries if the
	dynamic relocation section will be discarded from output.
	(_bfd_mips_elf_finish_dynamic_sections) <DT_RELSZ>: Assert that
	the dynamic relocation section will be retained in output.

	ld/
	* testsuite/ld-elf/reloc-discard.d: New test.
	* testsuite/ld-elf/reloc-discard.ld: New test linker script.
	* testsuite/ld-elf/reloc-discard.s: New test source.