RFR: 8188102: [JVMCI] Convert special JVMCI oops in nmethod to jweak values

[Doug Simon]

Please review this change that converts the JVMCI-specific object references in nmethod from oops to weak values. This removes GC API extensions added purely for these fields (e.g. so that G1 can insert it into the right remembered set, and when unloading an nmethod, to go and remove the nmethod from that remembered set).

Testing: I've run the Graal unit tests (mx unittest --verbose --gc-after-test -Xlog:class+unload=trace) which trigger a lot of nmethod unloading.

https://bugs.openjdk.java.net/browse/JDK-8188102
http://cr.openjdk.java.net/~dnsimon/8188102/