[foreign-memaccess] RFR: JDK-8242048: Add API point to get raw long value out of a MemoryAddress

[Maurizio Cimadamore]

This simple patch adds an extra API to retrieve the long value of an address; the method throws UOE if the address is
heap-based. I have not added dedicated tests since this method is used pretty much everywhere (see MemoryHandles).

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Commit messages:
 - * Fix tests
 - * First stab at adding raw address value

Changes: https://git.openjdk.java.net/panama-foreign/pull/88/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/88/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8242048
  Stats: 53 lines in 6 files changed: 15 ins; 31 del; 7 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/88.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/88/head:pull/88

PR: https://git.openjdk.java.net/panama-foreign/pull/88