[foreign-memaccess+abi] RFR: 8266626: Check that the target address of a native call is not NULL [v3]

[Jorn Vernee]

> This patch adds a check when unboxing the target address of a native call to check that the address is not `NULL`. Additionally, it adds checks earlier when an address is specified with a linking request, to check that that address is not `NULL`.
> 
> I didn't see any regressions on the CallOverhead* benchmarks.
> 
> Thanks,
> Jorn

Jorn Vernee has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge branch 'foreign-memaccess+abi' into Check_NULL
 - - Merge check symbol code
   - add missing documentation
 - Add tests
 - WIP - Add NULL checks for target addresses (need tests)

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Changes: https://git.openjdk.java.net/panama-foreign/pull/530/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=530&range=02
  Stats: 97 lines in 8 files changed: 86 ins; 4 del; 7 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/530.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/530/head:pull/530

PR: https://git.openjdk.java.net/panama-foreign/pull/530