ELF: Resolve _end symbols correctly.

[pcc]

Summary

Fix a bug introduced in r297313 which caused them to resolve to the end
of the ELF header in PIEs and DSOs.

lld/ELF/Writer.cpp

 }
 
 // The linker is expected to define SECNAME_start and SECNAME_end
 // symbols for a few sections. This function defines them.
 template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
   auto Define = [&](StringRef Start, StringRef End, OutputSection *OS) {
     // These symbols resolve to the image base if the section does not exist.
     // A special value -1 indicates end of the section.
-    if (!OS && Config->pic())
+    if (OS) {
+      addOptionalRegular<ELFT>(Start, OS, 0);
+      addOptionalRegular<ELFT>(End, OS, -1);
+    } else {
+      if (Config->pic())
         OS = Out::ElfHeader;
       addOptionalRegular<ELFT>(Start, OS, 0);
-    addOptionalRegular<ELFT>(End, OS, OS ? -1 : 0);
+      addOptionalRegular<ELFT>(End, OS, 0);
+    }
   };
 
   Define("__preinit_array_start", "__preinit_array_end", Out::PreinitArray);
   Define("__init_array_start", "__init_array_end", Out::InitArray);
   Define("__fini_array_start", "__fini_array_end", Out::FiniArray);
 
   if (OutputSection *Sec = findSection(".ARM.exidx"))
     Define("__exidx_start", "__exidx_end", Sec);

lld/test/ELF/pre_init_fini_array_missing.s

 // RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 // RUN: ld.lld %t -o %t2
 // RUN: llvm-objdump -d %t2 | FileCheck %s
+// RUN: ld.lld -pie %t -o %t3
+// RUN: llvm-objdump -d %t3 | FileCheck --check-prefix=PIE %s
 // REQUIRES: x86
 
 .globl _start
 _start:
   call __preinit_array_start
   call __preinit_array_end
   call __init_array_start
   call __init_array_end

 // CHECK: Disassembly of section .text:
 // CHECK-NEXT:  _start:
 // CHECK-NEXT:   201000:    e8 fb ef df ff     callq    -2101253
 // CHECK-NEXT:   201005:    e8 f6 ef df ff     callq    -2101258
 // CHECK-NEXT:   20100a:    e8 f1 ef df ff     callq    -2101263
 // CHECK-NEXT:   20100f:    e8 ec ef df ff     callq    -2101268
 // CHECK-NEXT:   201014:    e8 e7 ef df ff     callq    -2101273
 // CHECK-NEXT:   201019:    e8 e2 ef df ff     callq    -2101278
+
+// In position-independent binaries, they resolve to the image base.
+
+// PIE:      Disassembly of section .text:
+// PIE-NEXT: _start:
+// PIE-NEXT:     1000:  e8 fb ef ff ff  callq -4101
+// PIE-NEXT:     1005:  e8 f6 ef ff ff  callq -4106
+// PIE-NEXT:     100a:  e8 f1 ef ff ff  callq -4111
+// PIE-NEXT:     100f:  e8 ec ef ff ff  callq -4116
+// PIE-NEXT:     1014:  e8 e7 ef ff ff  callq -4121
+// PIE-NEXT:     1019:  e8 e2 ef ff ff  callq -4126