Update code gen in CallGeneratorHelper

The code in test/jdk/java/foreign/CallGeneratorHelper used to generate native test libraries is out of date. There are several issues:

- We no longer generate header files, so the code for that is dead
- The code was never updated to generate the *Stack.c test files
- The code was never updated to generate the shared.h file

The native test libraries that are generated closely correspond to the test cases that are generated by the actual TestNG tests (the former uses the same DataProvier as the latter). So, I think it is worthwhile to keep around/update the native test file generation code, in case we ever need to tweak the test case generation logic (which would then require re-generating the native test files).