RFR: 8330261: Clean up linking steps [v4]

[Magnus Ihse Bursie]

> Instead of executing code directly in Link.gmk, we set variables that are then evaluated to get the code we want. This is non-transparent and makes it unnecessarily hard to work with the linking code base. 
> 
> This PR also contains some additional code cleanup/fixes.

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  MACOSX_CODESIGN_MODE was not always properly set

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18783/files
  - new: https://git.openjdk.org/jdk/pull/18783/files/65765e49..df3f64a2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18783&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18783&range=02-03

  Stats: 7 lines in 1 file changed: 2 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18783.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18783/head:pull/18783

PR: https://git.openjdk.org/jdk/pull/18783