[Lilliput] (AArch64) Use tbz instead of tst and br in load_klass()

In the implementation of load_klass(), we have a test-and-branch idiom:

tst(dst, markWord::monitor_value);
br(Assembler::EQ, fast);

We can make this better:

tbz(dst, exact_log2(markWord::monitor_value), fast);

This is not only smaller and perhaps a little faster, it also has the advantage that it doesn't touch the condition flags. This is relevant in at least one place: in verify_oop() the condition flag is alive across the load_klass() call and one jtreg test (runtime/CheckUnhandledOops/TestVerifyOops.java) is failing because of that.