[mips] Recognise the triple used by Debian stretch for mips64el.

[dsanders]

Summary

The triple used for this distribution is mips64el-linux-gnuabi64.

Please note that this is unrelated to my other work that's under review at the
moment. This is about recursing LLVM on a new port of Debian.

[sdardis]

LGTM.

include/llvm/ADT/Triple.h

enum OSType {
     WatchOS,    // Apple watchOS
     Mesa3D,
     LastOSType = Mesa3D
   };
   enum EnvironmentType {
     UnknownEnvironment,
 
     GNU,
+    GNUABI64,
     GNUEABI,
     GNUEABIHF,
     GNUX32,
     CODE16,
     EABI,
     EABIHF,
     Android,
     Musl,
public:
   }
 
   bool isOSIAMCU() const {
     return getOS() == Triple::ELFIAMCU;
   }
 
   bool isGNUEnvironment() const {
     EnvironmentType Env = getEnvironment();
-    return Env == Triple::GNU || Env == Triple::GNUEABI ||
-           Env == Triple::GNUEABIHF || Env == Triple::GNUX32;
+    return Env == Triple::GNU || Env == Triple::GNUABI64 ||
+           Env == Triple::GNUEABI || Env == Triple::GNUEABIHF ||
+           Env == Triple::GNUX32;
   }
 
   /// Checks if the environment could be MSVC.
   bool isWindowsMSVCEnvironment() const {
     return getOS() == Triple::Win32 &&
            (getEnvironment() == Triple::UnknownEnvironment ||
             getEnvironment() == Triple::MSVC);
   }

lib/Support/Triple.cpp

const char *Triple::getOSTypeName(OSType Kind) {
 
   llvm_unreachable("Invalid OSType");
 }
 
 const char *Triple::getEnvironmentTypeName(EnvironmentType Kind) {
   switch (Kind) {
   case UnknownEnvironment: return "unknown";
   case GNU: return "gnu";
+  case GNUABI64: return "gnuabi64";
   case GNUEABIHF: return "gnueabihf";
   case GNUEABI: return "gnueabi";
   case GNUX32: return "gnux32";
   case CODE16: return "code16";
   case EABI: return "eabi";
   case EABIHF: return "eabihf";
   case Android: return "android";
   case Musl: return "musl";
return StringSwitch<Triple::OSType>(OSName)
     .StartsWith("mesa3d", Triple::Mesa3D)
     .Default(Triple::UnknownOS);
 }
 
 static Triple::EnvironmentType parseEnvironment(StringRef EnvironmentName) {
   return StringSwitch<Triple::EnvironmentType>(EnvironmentName)
     .StartsWith("eabihf", Triple::EABIHF)
     .StartsWith("eabi", Triple::EABI)
+    .StartsWith("gnuabi64", Triple::GNUABI64)
     .StartsWith("gnueabihf", Triple::GNUEABIHF)
     .StartsWith("gnueabi", Triple::GNUEABI)
     .StartsWith("gnux32", Triple::GNUX32)
     .StartsWith("code16", Triple::CODE16)
     .StartsWith("gnu", Triple::GNU)
     .StartsWith("android", Triple::Android)
     .StartsWith("musleabihf", Triple::MuslEABIHF)
     .StartsWith("musleabi", Triple::MuslEABI)