[foreign-memaccess+abi] RFR: 8292174: Add unaligned layouts to ValueLayout

[Per Minborg]

The layouts are placed in a special `Unaligned` class. This prevents the classes to be seen on the top level (for less cluttering) and allows lazy initialization of items that are likely not so much used.

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Commit messages:
 - Add unaligned value layouts

Changes: https://git.openjdk.org/panama-foreign/pull/705/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=705&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292174
  Stats: 120 lines in 1 file changed: 120 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/705.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/705/head:pull/705

PR: https://git.openjdk.org/panama-foreign/pull/705