Use default target triple, not process triple when unit testing MCJIT

[broadwaylamb]

Summary

sys::getProcessTriple() doesn't play well with LLVM built for cross-compilation.

I'm not sure whether this is the right solution, but to me it makes more sense, besides, it works on my machine™®

llvm/unittests/ExecutionEngine/MCJIT/MCJITTestAPICommon.h

if (!ArchSupportsMCJIT() || !OSSupportsMCJIT()) \
       return; \
   while(0)
 
 namespace llvm {
 
 class MCJITTestAPICommon {
 protected:
   MCJITTestAPICommon()
-    : HostTriple(sys::getProcessTriple())
+    : HostTriple(sys::getDefaultTargetTriple())
   {
     InitializeNativeTarget();
     InitializeNativeTargetAsmPrinter();
 
     // FIXME: It isn't at all clear why this is necesasry, but without it we
     // fail to initialize the AssumptionCacheTracker.
     initializeAssumptionCacheTrackerPass(*PassRegistry::getPassRegistry());