[ELF] Write R_X86_64_IRELATIVE addends with -z rel

[arichardson]

Summary

I found this missing case with the new --check-dynamic-relocation flag
while running the lld tests with --apply-dynamic-relocs enabled by default.

This also fixes a broken CHECK in lld/test/ELF/x86-64-gotpc-relax.s:
The test wasn't using CHECK-NEXT, so it was passing despite the output
actually containing relocations. I am not sure when this changed, but I
think this behaviour is correct.

Found with D101450 + enabling --apply-dynamic-relocs by default.

[MaskRay]

Looks great!

lld/ELF/Arch/X86_64.cpp

 public:
   X86_64();
   int getTlsGdRelaxSkip(RelType type) const override;
   RelExpr getRelExpr(RelType type, const Symbol &s,
                      const uint8_t *loc) const override;
   RelType getDynRel(RelType type) const override;
   void writeGotPltHeader(uint8_t *buf) const override;
   void writeGotPlt(uint8_t *buf, const Symbol &s) const override;
+  void writeIgotPlt(uint8_t *buf, const Symbol &s) const override;
   void writePltHeader(uint8_t *buf) const override;
   void writePlt(uint8_t *buf, const Symbol &sym,
                 uint64_t pltEntryAddr) const override;
   void relocate(uint8_t *loc, const Relocation &rel,
                 uint64_t val) const override;
   int64_t getImplicitAddend(const uint8_t *buf, RelType type) const override;
   void applyJumpInstrMod(uint8_t *loc, JumpModType type,
                          unsigned size) const override;
void X86_64::writeGotPltHeader(uint8_t *buf) const {
   write64le(buf, mainPart->dynamic->getVA());
 }
 
 void X86_64::writeGotPlt(uint8_t *buf, const Symbol &s) const {
   // See comments in X86::writeGotPlt.
   write64le(buf, s.getPltVA() + 6);
 }
 
+void X86_64::writeIgotPlt(uint8_t *buf, const Symbol &s) const {
+  // An x86 entry is the address of the ifunc resolver function (for -z rel).
+  if (config->writeAddends)
+    write64le(buf, s.getVA());
+}
+
 void X86_64::writePltHeader(uint8_t *buf) const {
   const uint8_t pltData[] = {
       0xff, 0x35, 0, 0, 0, 0, // pushq GOTPLT+8(%rip)
       0xff, 0x25, 0, 0, 0, 0, // jmp *GOTPLT+16(%rip)
       0x0f, 0x1f, 0x40, 0x00, // nop
   };
   memcpy(buf, pltData, sizeof(pltData));
   uint64_t gotPlt = in.gotPlt->getVA();

lld/test/ELF/x86-64-gotpc-relax.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -relax-relocations -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o -o %t1 --no-apply-dynamic-relocs
+# RUN: llvm-readobj -x .got.plt -r %t1 | FileCheck --check-prefixes=RELOC,NO-APPLY-DYNAMIC-RELOCS %s
+# RUN: ld.lld %t.o -o %t1 --apply-dynamic-relocs
+# RUN: llvm-readobj -x .got.plt -r %t1 | FileCheck --check-prefixes=RELOC,APPLY-DYNAMIC-RELOCS %s
 # RUN: ld.lld %t.o -o %t1
-# RUN: llvm-readobj -r %t1 | FileCheck --check-prefix=RELOC %s
 # RUN: llvm-objdump -d %t1 | FileCheck --check-prefix=DISASM %s
 
-## There is no relocations.
-# RELOC:    Relocations [
-# RELOC:    ]
+## There is one R_X86_64_IRELATIVE relocations.
+# RELOC-LABEL: Relocations [
+# RELOC-NEXT:    Section (1) .rela.dyn {
+# RELOC-NEXT:      0x202220 R_X86_64_IRELATIVE - 0x201172
+# RELOC-NEXT:    }
+# RELOC-NEXT:  ]
+# RELOC-LABEL: Hex dump of section '.got.plt':
+# NO-APPLY-DYNAMIC-RELOCS-NEXT:  0x00202220 00000000 00000000
+# APPLY-DYNAMIC-RELOCS-NEXT:     0x00202220 72112000 00000000
+# RELOC-EMPTY:
 
 # 0x201173 + 7 - 10 = 0x201170
 # 0x20117a + 7 - 17 = 0x201170
 # 0x201181 + 7 - 23 = 0x201171
 # 0x201188 + 7 - 30 = 0x201171
 # DISASM:      Disassembly of section .text:
 # DISASM-EMPTY:
 # DISASM-NEXT: <foo>: