Provide a stub aeabi.read_tp on other archs

Using "#if defined(...)" inside the test body seems to be the convention.

Bug: none
Test: bionic-unit-tests
Change-Id: Icb348a1184e630c5e199129bbe2090fd1e490fa5

tests/__aeabi_read_tp_test.cpp

@@ -32,7 +32,12 @@
 
 #if defined(__arm__)
 extern "C" void* __aeabi_read_tp();
-TEST(aeabi, read_tp) {
-  ASSERT_EQ(__aeabi_read_tp(), static_cast<void*>(__get_tls()));
-}
 #endif
+
+TEST(aeabi, read_tp) {
+#if defined(__arm__)
+  ASSERT_EQ(__aeabi_read_tp(), static_cast<void*>(__get_tls()));
+#else
+  GTEST_LOG_(INFO) << "__aeabi_read_tp is only available on arm32.\n";
+#endif
+}