[foreign-memaccess+abi] RFR: 8299181: PaddingLayout unable to return byteAlignment value [v2]

[Per Minborg]

> This PR proposes bit-aligning `PaddingLayout` to 8 by default as opposed to the current  value of 1.
> 
> More generally, the PR restricts `PaddingLayout` bit-alignment to non-negative values that are an even multiple of 8 (i.e. 0, 8, 16, ...). There are no mechanism to derive `VarHandle` objects for sub-byte access anyhow and trying to obtain a miss-aligned `VarHandle` would throw an exception. 
> 
> If integrated, all `MemoryLayout` types would be byte-aligned.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Improve after comments

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/766/files
  - new: https://git.openjdk.org/panama-foreign/pull/766/files/fc444c0e..e3f7fcc4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=766&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=766&range=00-01

  Stats: 26 lines in 2 files changed: 5 ins; 18 del; 3 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/766.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/766/head:pull/766

PR: https://git.openjdk.org/panama-foreign/pull/766