[foreign-memaccess+abi] RFR: 8291873: Implement return value normalization in Java

[Jorn Vernee]

This patch moves value normalization code from the downcall stub (where it was hard coded) to Java, and also adds it for upcall arguments where it was missing.

There is a slight change in behavior with this. The previous hard-coded conversion from a native value to Java `boolean` checked if the least significant byte was non-zero, i.e. `boolean result = value & 0xFF != 0`. While the new conversion only looks at the least significant bit, i.e. `boolean result = value & 0b1 != 0`. I think the new behavior is more correct. It means that the Linker will just do the "dumb" thing when mapping anything to `boolean`, and the native representation is really expected to be like a Java `boolean` i.e. only using the least significant bit. On the other hand I think it means that e.g. jextract will have to map the C `bool` as `byte` and then do a non-zero check to convert to `boolean`.

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Commit messages:
 - polish whitespace
 - fix
 - better test
 - add test, remove normalization code in downcall stub

Changes: https://git.openjdk.org/panama-foreign/pull/720/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=720&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291873
  Stats: 365 lines in 8 files changed: 316 ins; 35 del; 14 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/720.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/720/head:pull/720

PR: https://git.openjdk.org/panama-foreign/pull/720