[MC] support TLSDESC and TLSCALL / GNU2 tls dialect

[davide]

Summary

Fixes longstanding PR16814.
For reference: http://www.fsfla.org/~lxoliva/writeups/TLS/RFC-TLSDESC-x86.txt

llvm/trunk/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp

case RT64_64:
       return ELF::R_X86_64_SIZE64;
     case RT64_32:
       return ELF::R_X86_64_SIZE32;
     case RT64_32S:
     case RT64_16:
     case RT64_8:
       llvm_unreachable("Unimplemented");
     }
+  case MCSymbolRefExpr::VK_TLSCALL:
+    return ELF::R_X86_64_TLSDESC_CALL;
+  case MCSymbolRefExpr::VK_TLSDESC:
+    return ELF::R_X86_64_GOTPC32_TLSDESC;
   case MCSymbolRefExpr::VK_TLSGD:
     checkIs32(Ctx, Loc, Type);
     return ELF::R_X86_64_TLSGD;
   case MCSymbolRefExpr::VK_GOTTPOFF:
     checkIs32(Ctx, Loc, Type);
     return ELF::R_X86_64_GOTTPOFF;
   case MCSymbolRefExpr::VK_TLSLD:
     checkIs32(Ctx, Loc, Type);

llvm/trunk/test/MC/ELF/relocation-tls.s

+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - | llvm-readobj -s -sr | FileCheck  %s
+
+// Test that we produce the correct relocation.
+
+        leaq    _ZL3ccc@TLSDESC(%rip), %rax
+        call    *_ZL3ccc@TLSCALL(%rax)
+        addq    %fs:0, %rax
+
+// CHECK: Section {
+// CHECK:   Index:
+// CHECK:   Name: .rela.text
+// CHECK-NEXT:   Type: SHT_RELA
+// CHECK-NEXT:   Flags [
+// CHECK-NEXT:   ]
+// CHECK-NEXT:   Address: 0x0
+// CHECK-NEXT:   Offset:
+// CHECK-NEXT:   Size:
+// CHECK-NEXT:   Link:
+// CHECK-NEXT:   Info:
+// CHECK-NEXT:   AddressAlignment: 8
+// CHECK-NEXT:   EntrySize: 24
+// CHECK-NEXT:   Relocations [
+// CHECK-NEXT:     0x3 R_X86_64_GOTPC32_TLSDESC _ZL3ccc 0xFFFFFFFFFFFFFFFC
+// CHECK-NEXT:     0x9 R_X86_64_TLSDESC_CALL _ZL3ccc 0x0
+// CHECK-NEXT:   ]
+// CHECK-NEXT: }