Add a test for the offset of dynamic relocations.

I found that a patch I am working on would have broken this and no
existing test found it.

llvm-svn: 265664

lld/test/ELF/rel-offset.s

@@ -0,0 +1,15 @@
+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+// RUN: ld.lld %t.o -o %t -shared
+// RUN: llvm-readobj -r  %t | FileCheck %s
+
+        .section        .data.foo,"aw",@progbits
+        .quad   foo
+
+        .section        .data.zed,"aw",@progbits
+        .quad   foo
+
+// CHECK:      Section ({{.*}}) .rela.dyn {
+// CHECK-NEXT:   0x2000 R_X86_64_64 foo 0x0
+// CHECK-NEXT:   0x2008 R_X86_64_64 foo 0x0
+// CHECK-NEXT: }