RFR: makefile changes for mach5 build

[Athijegannathan Sundararajan]

backporting the same changes from jdk19 branch

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Commit messages:
 - More makefile tweaks
 - Fixes needed for mach5

Changes: https://git.openjdk.java.net/jextract/pull/47/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=47&range=00
  Stats: 185 lines in 4 files changed: 127 ins; 1 del; 57 mod
  Patch: https://git.openjdk.java.net/jextract/pull/47.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/47/head:pull/47

PR: https://git.openjdk.java.net/jextract/pull/47