[mips][ias] Support .dtprel[d]word and .tprel[d]word directives

Assembler directives .dtprelword, .dtpreldword, .tprelword, and
.tpreldword generates relocations R_MIPS_TLS_DTPREL32, R_MIPS_TLS_DTPREL64,
R_MIPS_TLS_TPREL32, and R_MIPS_TLS_TPREL64 respectively.

The main motivation for this patch is to be able to write test cases
for checking correctness of the LLD linker's behaviour.

Differential Revision: https://reviews.llvm.org/D23669

llvm-svn: 279439
diff --git a/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp b/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
index 3ee0cd9..38b11f7 100644
--- a/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
+++ b/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
@@ -59,6 +59,10 @@
   case Mips::fixup_MIPS_PCLO16:
     Value &= 0xffff;
     break;
+  case FK_DTPRel_4:
+  case FK_DTPRel_8:
+  case FK_TPRel_4:
+  case FK_TPRel_8:
   case FK_GPRel_4:
   case FK_Data_4:
   case FK_Data_8: