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.

bfd/elfnn-loongarch.c

@@ -1368,6 +1368,8 @@ allocate_dynrelocs (struct elf_link_hash_entry *h, void *inf)
 
   for (p = h->dyn_relocs; p != NULL; p = p->next)
     {
+      if (discarded_section (p->sec))
+	continue;
       asection *sreloc = elf_section_data (p->sec)->sreloc;
       sreloc->size += p->count * sizeof (ElfNN_External_Rela);
     }

ld/testsuite/ld-loongarch-elf/ld-loongarch-elf.exp

@@ -147,3 +147,4 @@ run_dump_test "underflow_b16"
 run_dump_test "underflow_b21"
 run_dump_test "underflow_b26"
 run_dump_test "underflow_pcrel20"
+run_dump_test "pie_discard"

ld/testsuite/ld-loongarch-elf/pie_discard.d

@@ -0,0 +1,10 @@
+#source: pie_discard.s
+#ld: -pie -e 0 -T pie_discard.t
+#readelf: -rW
+
+#...
+Relocation section '\.rela\.dyn' .* 1 .*
+#...
+.*R_LARCH_RELATIVE.*
+#pass
+

ld/testsuite/ld-loongarch-elf/pie_discard.s

@@ -0,0 +1,9 @@
+  .text
+  .global sym
+sym: nop
+
+  .section .data,"aw"
+  .dword sym
+
+  .section .discard,"aw"
+  .dword sym

ld/testsuite/ld-loongarch-elf/pie_discard.t

@@ -0,0 +1,9 @@
+SECTIONS
+{
+  . = SEGMENT_START("text-segment", 0) + SIZEOF_HEADERS;
+  .rela.dyn : { *(.rela.*) }
+  .text : { *(.text) }
+  . = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));
+  .data : { *(.data) }
+  /DISCARD/ : { *(.discard) }
+}