[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/MipsMCAsmInfo.cpp b/llvm/lib/Target/Mips/MCTargetDesc/MipsMCAsmInfo.cpp
index 2c01f91..5a394fe8 100644
--- a/llvm/lib/Target/Mips/MCTargetDesc/MipsMCAsmInfo.cpp
+++ b/llvm/lib/Target/Mips/MCTargetDesc/MipsMCAsmInfo.cpp
@@ -45,6 +45,10 @@
   ZeroDirective               = "\t.space\t";
   GPRel32Directive            = "\t.gpword\t";
   GPRel64Directive            = "\t.gpdword\t";
+  DTPRel32Directive           = "\t.dtprelword\t";
+  DTPRel64Directive           = "\t.dtpreldword\t";
+  TPRel32Directive            = "\t.tprelword\t";
+  TPRel64Directive            = "\t.tpreldword\t";
   UseAssignmentForEHBegin = true;
   SupportsDebugInformation = true;
   ExceptionsType = ExceptionHandling::DwarfCFI;