RFR: 8301834: Templated Buffer classes leave a lot of empty lines in the generated source

[Jaikiran Pai]

Can I please get a review for this change which proposes to fix the issue noted in https://bugs.openjdk.org/browse/JDK-8301834?

Some classes in `java.nio` package are generated from template files, during the build. The template files are processed by a build tool implemented by a Java class `make/jdk/src/classes/build/tools/spp/Spp.java`. This template processing tool allows for an (optional) parameter called `-nel` which as per its documentation:

>  If -nel is declared then empty lines will not be substituted for lines of
>  text in the template that do not appear in the output.

Various places in the JDK build where this tool is used to generate source from template files, already use the `-nel` option to not generate the empty lines in the source files. However, the `GensrcBuffer.gmk` which generates the source for `java.nio` classes doesn't use this option. The commit in this PR adds this option when generating the `java.nio` classes.

Existing tests in `test/jdk/java/nio` continue to pass after this change. I've always checked the generated content and compared it with the older versions to verify that these empty lines no longer appear in these generated classes.

Additional `tier` testing has been triggered to make sure no regression is introduced.

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Commit messages:
 - 8301834: Templated Buffer classes leave a lot of empty lines in the generated source

Changes: https://git.openjdk.org/jdk/pull/12431/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12431&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301834
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12431.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12431/head:pull/12431

PR: https://git.openjdk.org/jdk/pull/12431