RFR: 8326509: Clean up JNIEXPORT in Hotspot after JDK-8017234

[Magnus Ihse Bursie]

Once [JDK-8017234](https://bugs.openjdk.org/browse/JDK-8017234) has been integrated, it is possible to do some cleanup. The goal of [JDK-8017234](https://bugs.openjdk.org/browse/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.

Note that this PR is [dependent on](https://mail.openjdk.org/pipermail/jdk-dev/2021-March/005232.html) https://github.com/openjdk/jdk/pull/17955.

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Depends on: https://git.openjdk.org/jdk/pull/17955

Commit messages:
 - 8326509: Clean up JNIEXPORT in Hotspot after JDK-8017234

Changes: https://git.openjdk.org/jdk/pull/17967/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17967&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326509
  Stats: 29 lines in 4 files changed: 0 ins; 20 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/17967.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17967/head:pull/17967

PR: https://git.openjdk.org/jdk/pull/17967