RFR: 8301753: AppendFile/WriteFile has differences between make 3.81 and 4+

[Magnus Ihse Bursie]

This is a retake on https://github.com/openjdk/jdk/pull/12461. Changes compared to the original patch:

Both WriteFile and AppendFile are fixed. 

It turned out we actually did a `$(strip)` on the input for the `$(file)` variant, so for consistency we should do that for the make 3.81 as well. Almost as a side-effect, this will also handle the problem of risking to add multiple newlines, since any trailing newlines will be removed by the strip, and then we can always append a newline.

Unit tests are added. I have verified functionality on make 3.81, 4.0 and 4.4.

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Commit messages:
 - 8301753: AppendFile/WriteFile has differences between make 3.81 and 4+

Changes: https://git.openjdk.org/jdk/pull/12605/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12605&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301753
  Stats: 58 lines in 2 files changed: 56 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12605.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12605/head:pull/12605

PR: https://git.openjdk.org/jdk/pull/12605