message: Fix llvm_unreachable in TestWriteMemory
  The test was hitting llvm_unreachable in
  Platform::GetSoftwareBreakpointTrapOpcode because it could not figure
  out the architecture of the process. Since that is not the purpose of
  the test, I change the test to use an explicit
  CreateTargetWithFileAndTargetTriple command to specify it.
diff: @@ -14,7 +14,7 @@ class TestWriteMemory(GDBRemoteTestBase):
  return "OK"
  self.server.responder = MyResponder()
  - target = self.dbg.CreateTarget('')
  + target = self.dbg.CreateTargetWithFileAndTargetTriple('', 'x86_64-pc-linux')
  process = self.connect(target)
  bp = target.BreakpointCreateByAddress(0x1000)
  