[foreign-jextract] RFR: 8257892: long double not handled by jextract

[Athijegannathan Sundararajan]

added UnsupportedLayouts and checking/warning all unsupported types uniformly

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Commit messages:
 - 8257892: long double not handled by jextract

Changes: https://git.openjdk.java.net/panama-foreign/pull/409/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=409&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257892
  Stats: 117 lines in 9 files changed: 104 ins; 5 del; 8 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/409.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/409/head:pull/409

PR: https://git.openjdk.java.net/panama-foreign/pull/409