[foreign-abi] RFR: AArch64 build fixes

[Jorn Vernee]

Hi,

This patch fixes several linker errors observed when building on aarch64, due to missing implementations of methods
added for the C2 down call intrinsics.

Currently compilation will just bail out, but I think another strategy is possible where we generate a call to the
fallback MethodHandle instead. This is just fixing the build.

Thanks,
Jorn

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Commit messages:
 - AArch64 build fixes

Changes: https://git.openjdk.java.net/panama-foreign/pull/238/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/238/webrev.00
  Stats: 29 lines in 5 files changed: 28 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/238.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/238/head:pull/238

PR: https://git.openjdk.java.net/panama-foreign/pull/238