ARM: suuport .tlsdescseq directive

This enhances the ARMAsmParser to handle .tlsdescseq directives.  This is a
slightly special relocation.  We must be able to generate them, but not consume
them in assembly.  The relocation is meant to assist the linker in generating a
TLS descriptor sequence.  The ELF target streamer is enhanced to append
additional fixups into the current segment and that is used to emit the new
R_ARM_TLS_DESCSEQ relocations.

llvm-svn: 200448
diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
index d402e6d..75bde8e 100644
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -187,6 +187,7 @@
   case VK_ARM_TLSLDO: return "tlsldo";
   case VK_ARM_TLSCALL: return "tlscall";
   case VK_ARM_TLSDESC: return "tlsdesc";
+  case VK_ARM_TLSDESCSEQ: return "tlsdescseq";
   case VK_PPC_LO: return "l";
   case VK_PPC_HI: return "h";
   case VK_PPC_HA: return "ha";