Wrong initialization of nmethod::_consts_offset for native nmethods

Symptom 1: Negative sizes of constants sections for native nmethods

Reproduce:

* Build with attached patch
* Run `java -Xcomp -Xlog:jit+compilation=debug -version`

The output will contain "NEGATIVE constants section size" for every native nmethod.

Symptom 2: Empty constants section in PrintAssembly output for (Loom) continuation
enter intrinsic even though, at least on PPC, it is not empty.

Analysis: JDK-6961697 [1] moved the constants section before the instruction section. This was
reflected in the initialization of nmethod::_consts_offset done by the nmethod
constructor for c1/c2 compiled nmethods. It was forgotten though to update the
constructor for native nmethods.

[1] Commit for JDK-6961697
    https://github.com/openjdk/jdk/commit/6a0c40f1fee892687ab615dd2566109e8204ec96