[RuntimeDyld] Add support for R_X86_64_PC8 relocation.

[davide]

This looks good but please wait for Lang's sign-off.

[lhames]

LGTM. Thanks Maksim!

lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.cpp

                  << format("%p\n", Section.Address + Offset));
     break;
   }
+  case ELF::R_X86_64_PC8: {
+    uint64_t FinalAddress = Section.LoadAddress + Offset;
+    int64_t RealOffset = Value + Addend - FinalAddress;
+    assert(isInt<8>(RealOffset));
+    int8_t TruncOffset = (RealOffset & 0xFF);
+    Section.Address[Offset] = TruncOffset;
+    break;
+  }
   case ELF::R_X86_64_PC32: {
     uint64_t FinalAddress = Section.LoadAddress + Offset;
     int64_t RealOffset = Value + Addend - FinalAddress;

test/ExecutionEngine/RuntimeDyld/X86/ELF_x64-64_PC8_relocations.s

+# RUN: llvm-mc -triple=x86_64-pc-linux -relocation-model=pic -filetype=obj -o %T/test_ELF_x86-64_PC8.o %s
+# RUN: llvm-rtdyld -triple=x86_64-pc-linux -verify -map-section test_ELF_x86-64_PC8.o,.text.bar=0x10000 -map-section test_ELF_x86-64_PC8.o,.text.baz=0x10040 %T/test_ELF_x86-64_PC8.o
+# RUN: llvm-rtdyld -triple=x86_64-pc-linux -verify -map-section test_ELF_x86-64_PC8.o,.text.baz=0x10000 -map-section test_ELF_x86-64_PC8.o,.text.bar=0x10040 %T/test_ELF_x86-64_PC8.o
+
+# Test that R_X86_64_PC8 relocation works.
+
+  .section .text.bar,"ax"
+  .align  16, 0x90
+  .type bar,@function
+bar:
+  retq
+.Ltmp1:
+  .size bar, .Ltmp1-bar
+
+  .section .text.baz,"ax"
+  .align  16, 0x90
+  .type baz,@function
+baz:
+  movq  %rdi, %rcx
+  jrcxz bar
+  retq
+.Ltmp2:
+  .size baz, .Ltmp2-baz
+
+
+  .section  ".note.GNU-stack","",@progbits