Clean up JNIEXPORT in Hotspot after JDK-8017234

Once JDK-8017234 has been integrated, it is possible to do some cleanup. The goal of JDK-8017234 was to not change any behavior, even if that behavior seemed odd.

Now let's try to fix that. We can:
a) remove JNIEXPORT from c2v functions.
b) make debug.cpp functions exported similarly on all platforms
c) remove JNIEXPORT from aarch64 asm debug function.