Fix string compression, add tests.

Fix String.compareTo() for interpreter; memcmp() does not
return the required result (only the right sign).
Fix x86-64 stub where the assembler silently accepted and
generated bad code for out-of-range JECXZ.
Add extensive tests for String.equals(), String.compareTo()
and String.indexOf().

Bug: 31040547
Test: Run ART test suite including interpreter tests on host and Nexus 9.
Test: Ditto with string compression enabled.
Change-Id: I21b7a74da8a577c8fbaf8d9225f048550236d414