RFR: 8264650: Cross-compilation to macos/aarch64

[Anton Kozlov]

Please review adding necessary flags for cross-compilation on macos/x86 targeting macos/aarch64.

All CFLAGS, CXXFLAGS, LDFLAGS need target CPU flag `-arch arm64`.
This patch adds the flag along e.g `-m64`.

Tested:
* cross-compilation on macos/x86 to macos/aarch64: configure --openjdk-target=aarch64-apple-darwin
* native compilation on macos/aarch64
* native compilation on macos/x86

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Commit messages:
 - Append the flag
 - macos/aarch64 flags for cross compilation

Changes: https://git.openjdk.java.net/jdk/pull/3325/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3325&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264650
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3325.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3325/head:pull/3325

PR: https://git.openjdk.java.net/jdk/pull/3325