RFR: 8326587: Split Link.gmk into a Windows and Unix part

[Magnus Ihse Bursie]

There is not much overlap on how linking is done on Windows on one hand, and on all Unix platforms on the other. This makes Link.gmk basically consists of two parts, each in it own half of if statements, and the few common parts are artificially shoehorned in to fit both sides.

The code will be much clearer if we just split this into two different files.

Note that this PR slightly collides with JDK-8326583 (https://github.com/openjdk/jdk/pull/17986). Whichever of this goes in first will mean that the other one needs to make some adaptations.

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Commit messages:
 - 8326587: Split Link.gmk into a Windows and Unix part

Changes: https://git.openjdk.org/jdk/pull/17987/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17987&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326587
  Stats: 659 lines in 13 files changed: 362 ins; 283 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/17987.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17987/head:pull/17987

PR: https://git.openjdk.org/jdk/pull/17987