[ELF] - Create _DYNAMIC symbol for dynamic output

lld needs to provide _DYNAMIC symbol when creating a shared library
both bfd and gold do that.

This should fix the https://llvm.org/bugs/show_bug.cgi?id=26732

Differential revision: http://reviews.llvm.org/D17607

llvm-svn: 262348
diff --git a/lld/test/ELF/gc-sections-local-sym.s b/lld/test/ELF/gc-sections-local-sym.s
index 5102e30..db9f4c37 100644
--- a/lld/test/ELF/gc-sections-local-sym.s
+++ b/lld/test/ELF/gc-sections-local-sym.s
@@ -21,7 +21,7 @@
 // CHECK-NEXT: AddressAlignment:
 // CHECK-NEXT: EntrySize:
 // CHECK-NEXT: SectionData (
-// CHECK-NEXT:   0000: 00666F6F 00                          |.foo.|
+// CHECK-NEXT:   0000: 00666F6F 005F4459 4E414D49 4300      |.foo._DYNAMIC.|
 // CHECK-NEXT: )
 
 // CHECK:      Symbols [
@@ -35,6 +35,15 @@
 // CHECK-NEXT:     Section: Undefined
 // CHECK-NEXT:   }
 // CHECK-NEXT:   Symbol {
+// CHECK-NEXT:     Name: _DYNAMIC
+// CHECK-NEXT:     Value: 0x1000
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Local
+// CHECK-NEXT:     Type: None
+// CHECK-NEXT:     Other: 2
+// CHECK-NEXT:     Section: .dynamic
+// CHECK-NEXT:   }
+// CHECK-NEXT:   Symbol {
 // CHECK-NEXT:     Name: foo
 // CHECK-NEXT:     Value:
 // CHECK-NEXT:     Size: