[ELF] - Use SignExtend when reading R_386_PC8, R_386_PC16 addends.

Previously we did not do that. For example, for R_386_PC8, 
0xFF addend was not treated as 0xFFFFFFFF(-1), 
but was 0x000000FF.

Recently added checks for R_386_PC8/R_386_PC16 failed because of calculation 
overflow as a result.

Differential revision: https://reviews.llvm.org/D29490

llvm-svn: 294289

lld/ELF/Target.cpp

@@ -491,11 +491,13 @@ int64_t X86TargetInfo::getImplicitAddend(const uint8_t *Buf,
   default:
     return 0;
   case R_386_8:
-  case R_386_PC8:
     return *Buf;
+  case R_386_PC8:
+    return SignExtend64<8>(*Buf);
   case R_386_16:
-  case R_386_PC16:
     return read16le(Buf);
+  case R_386_PC16:
+    return SignExtend64<16>(read16le(Buf));
   case R_386_32:
   case R_386_GOT32:
   case R_386_GOT32X:

lld/test/ELF/i386-pc8-pc16-addend.s

@@ -0,0 +1,17 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i386-pc-linux-gnu %s -o %t1.o
+
+# RUN: ld.lld %t1.o -o %t.out
+# RUN: llvm-objdump -s -t %t.out | FileCheck %s
+# CHECK:      Contents of section .text:
+# CHECK-NEXT:  11000 020000 
+## 0x11003 - 0x11000 + addend(-1) = 0x02
+## 0x11003 - 0x11001 + addend(-2) = 0x0000
+# CHECK: SYMBOL TABLE:
+# CHECK: 00011003 .und
+
+.byte  und-.-1
+.short und-.-2
+
+.section .und, "ax"
+und: