RFR: 8190415: [JVMCI] JVMCIRuntime::adjust_comp_level must not swallow ThreadDeath

[Doug Simon]

Please review this change that fixes a JVMCI code path that was swallowing ThreadDeath exceptions and thus preventing Thread.stop from working as intended.
The webrev also contains some minor unrelated cleanup to mx_jvmci.py needed for supporting the consolidated repo.

The internal test that caught this problem is now passing.

https://bugs.openjdk.java.net/browse/JDK-8190415
http://cr.openjdk.java.net/~dnsimon/8190415/

-Doug