RFR: 8294006: Avoid hardcoding object file suffixes in make

[Julian Waters]

Replaces hardcoding of object file suffixes in make, to ensure they are always reliably set from a single place in autoconf.

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Commit messages:
 - Launcher.gmk
 - JvmMapfile.gmk
 - NativeCompilation.gmk

Changes: https://git.openjdk.org/jdk/pull/10341/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10341&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294006
  Stats: 9 lines in 3 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/10341.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10341/head:pull/10341

PR: https://git.openjdk.org/jdk/pull/10341