Avoid unused space in .rela.dyn if sec was discarded

Avoid unused space in .rela.dyn if sec was discarded

The relsec size is still increased although sec is discarded, which
cause a lot of unused space allocated. Avoid size increased if sec
was discarded.

bfd/ChangeLog:

	* bfd/elfnn-loongarch.c: (allocate_dynrelocs): Do not increase
	sreloc size when discarded_section.

ld/ChangeLog:

	* ld/testsuite/ld-loongarch-elf/ld-loongarch-elf.exp: Add test.
	* ld/testsuite/ld-loongarch-elf/pie_discard.d: New test.
	* ld/testsuite/ld-loongarch-elf/pie_discard.s: New test.
	* ld/testsuite/ld-loongarch-elf/pie_discard.t: New test.