[RISCV] Enable __int128_t and __uint128_t through clang flag

Summary:
If the flag -fforce-enable-int128 is passed, it will enable support for __int128_t and __uint128_t types.
This flag can then be used to build compiler-rt for RISCV32.

Reviewers: asb, kito-cheng, apazos, efriedma

Reviewed By: asb, efriedma

Subscribers: shiva0217, efriedma, jfb, dschuff, sdardis, sbc100, jgravelle-google, aheejin, rbar, johnrusso, simoncook, jordy.potman.lists, sabuasal, niosHD, cfe-commits

Differential Revision: https://reviews.llvm.org/D43105

llvm-svn: 326045
diff --git a/clang/lib/Basic/Targets/Mips.h b/clang/lib/Basic/Targets/Mips.h
index 24f0186..ff9e790 100644
--- a/clang/lib/Basic/Targets/Mips.h
+++ b/clang/lib/Basic/Targets/Mips.h
@@ -392,7 +392,9 @@
     return llvm::makeArrayRef(NewABIRegAliases);
   }
 
-  bool hasInt128Type() const override { return ABI == "n32" || ABI == "n64"; }
+  bool hasInt128Type() const override {
+    return (ABI == "n32" || ABI == "n64") || getTargetOpts().ForceEnableInt128;
+  }
 
   bool validateTarget(DiagnosticsEngine &Diags) const override;
 };