x86: Fix CriticalNative argument spilling in JNI stubs.

Do not move incoming stack arguments before we have spilled
the scratch register ECX.

Fix JniCompilerTest to initialize classes early so that we
really test what we were supposed to. This exposed the x86
bug fixed here.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 172332525
Change-Id: I7c06c8ccf18f5f66772c70f6a9a9a314668ce70d
2 files changed