[mips][ias] Enable IAS by default for N64 on Debian mips64el.

[dsanders]

Summary

Unfortunately we can't enable it for all N64 because it is not yet possible to
distinguish N32 from N64 from the triple on other environments.

N64 has been confirmed to produce identical (within reason) objects to GAS
during stage 2 of compiler recursion on N64-abi Fedora. Unfortunately,
Fedora's triples do not distinguish N32 from N64 so I can't enable it by
default there. I'm currently repeating this testing for Debian mips64el but
it's very unlikely to produce a different result.

[sdardis]

LGTM.

[sdardis]

Yes, I think you should also reflect mpf's comment here too.

[sdardis]

Actually, seeing your reply on the related patch, keep the comment as it reflects trunk.

[sdardis]

Committed as rL281610.

lib/Driver/ToolChains.cpp

bool Generic_GCC::IsIntegratedAssemblerDefault() const {
   case llvm::Triple::thumbeb:
   case llvm::Triple::ppc:
   case llvm::Triple::ppc64:
   case llvm::Triple::ppc64le:
   case llvm::Triple::systemz:
   case llvm::Triple::mips:
   case llvm::Triple::mipsel:
     return true;
+  case llvm::Triple::mips64:
+  case llvm::Triple::mips64el:
+    // Enabled for Debian mips64/mips64el only. Other targets are unable to
+    // distinguish N32 from N64.
+    if (getTriple().getEnvironment() == llvm::Triple::GNUABI64)
+      return true;
+    return false;
   default:
     return false;
   }
 }
 
 /// \brief Helper to add the variant paths of a libstdc++ installation.
 bool Generic_GCC::addLibStdCXXIncludePaths(
     Twine Base, Twine Suffix, StringRef GCCTriple, StringRef GCCMultiarchTriple,