ART: Actually run tests cross arch for oatdump

To test the "other" image, we need to give an --instruction-set
parameter. Without one, the current runtime architecture is
implicitly used.

Bug: 27824283
Change-Id: I9887fddee9f956ef40b40699cc6fe3378eb40808
diff --git a/oatdump/oatdump_test.cc b/oatdump/oatdump_test.cc
index 3e420ad..5728916 100644
--- a/oatdump/oatdump_test.cc
+++ b/oatdump/oatdump_test.cc
@@ -69,6 +69,7 @@
       exec_argv.push_back("--output=" + core_oat_location_ + ".symbolize");
     } else if (mode == kModeArt) {
       exec_argv.push_back("--image=" + core_art_location_);
+      exec_argv.push_back("--instruction-set=" + std::string(GetInstructionSetString(kRuntimeISA)));
       exec_argv.push_back("--output=/dev/null");
     } else {
       CHECK_EQ(static_cast<size_t>(mode), static_cast<size_t>(kModeOat));