RFR: 8308992: New test TestHFA fails with zero

[Jorn Vernee]

The issue is that the fallback linker uses `copyFrom` when copying a by-value struct argument to an internal buffer, without first adjusting the size of the argument segment. This means that if the argument segment is 'too large' (i.e. larger than the layout it was linked with) we fail with an exception since the internal buffer is too small for the entire argument segment to be copied into.

The fix is simply to use an exactly-sized copy instead, just like we do in other linker implementations. (The argument segment being too large is fine, all we care about is that it's large enough).

Testing: jdk-tier5

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Commit messages:
 - Use exactly-sized copy in fallback linker for by-value structs

Changes: https://git.openjdk.org/jdk/pull/14215/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14215&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308992
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14215.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14215/head:pull/14215

PR: https://git.openjdk.org/jdk/pull/14215