ARM: support TLS descriptor relocations

Add support for tlsdesc relocations which are part of the ABI, marked as
experimental.  These relocations permit the linker to perform TLS reference
optimizations.

llvm-svn: 200447
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
index 08c0402..5f719d5 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
@@ -266,6 +266,9 @@
       case MCSymbolRefExpr::VK_ARM_TLSCALL:
         Type = ELF::R_ARM_TLS_CALL;
         break;
+      case MCSymbolRefExpr::VK_ARM_TLSDESC:
+        Type = ELF::R_ARM_TLS_GOTDESC;
+        break;
       }
       break;
     case ARM::fixup_arm_ldst_pcrel_12: