[foreign-memaccess] RFR: 8246053: Add a mask for default access modesAdd a default access mode flag

[Maurizio Cimadamore]

This is a simple change which adds a 'default' access mask. This was pointed out during CSR review, and I realized that
many tests were also using it. It can also be handy when creating more restricted masks (e.g. start with default and
remove one or two access modes).

I've updated tests, documentation and some of the impl to use the new mask accordingly.

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Commit messages:
 - Add a default access mode flag

Changes: https://git.openjdk.java.net/panama-foreign/pull/188/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/188/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246053
  Stats: 56 lines in 6 files changed: 15 ins; 10 del; 31 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/188.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/188/head:pull/188

PR: https://git.openjdk.java.net/panama-foreign/pull/188